To expand on this answer:
Write a Groovy template for Email Ext plugin instead of Jelly template. 
In Editable Email Notification content 
- set content type to "HTML" or "Both HTML and Plain Text"
 
and include the groovy script like this:
${SCRIPT, template="test.groovy"}
 
put the groovy script in email-templates home e.g. 
/var/lib/jenkins/email-templates.  see below test.groovy.
 
In the example below every test is iterated by getting each of these objects: '''junitResult.getChildren()'''. If one desired to iterate only failed tests then junitResult.getFailedTests() could be used. See the hudson.tasks.junit.TestResult API: http://hudson-ci.org/javadoc/hudson/tasks/junit/PackageResult.html  also see  http://hudson-ci.org/javadoc/hudson/model/Build.html
Collection<ClassResult> getChildren()
List<CaseResult>    getFailedTests()
Example/template from email-ext-plugin can be seen here:
https://github.com/jenkinsci/email-ext-plugin/blob/master/src/main/resources/hudson/plugins/emailext/templates/groovy-html.template
This example shows summary test result and table for results for each test suite and individual test. test.groovy:
<html>
<body>
<%
    import hudson.model.*
    def build = Thread.currentThread().executable
    def buildNumber = build.number
    def buildNumHash = build.getDisplayName()
    def testCount = "0"
    def testPassed = "0"
    def testFailed = "0"
    def testSkipped = "0"
    def buildDuration = "0"
    if(build.testResultAction) {
        def testResult = build.testResultAction
        testCount = String.format("%d",(testResult.totalCount))
        testPassed = String.format("%d",(testResult.result.passCount))
        testFailed = String.format("%d",(testResult.result.failCount))
        testSkipped = String.format("%d",(testResult.result.skipCount))
        testDuration = String.format("%.2f",(testResult.result.duration ))
    }
    def workspace = build.getEnvVars()["WORKSPACE"]
    def buildName = build.getEnvVars()["JOB_NAME"]
    def BUILD_STATUS = build.getEnvVars()["BUILD_STATUS"]
    def BUILD_URL = build.getEnvVars()["BUILD_URL"]
    def testResult = hudson.tasks.junit.TestResult
    def testResult2 = build.getAction(hudson.tasks.junit.TestResultAction.class)
%>
start test.groovy <br><br>
<b>TEST RESULT:</b> $testCount total, <b>$testPassed pass</b>, <b>$testFailed fail</b>, $testSkipped skip.<br>
Workspace : $workspace<br>
Project Name : $buildName $buildNumHash<br><br>
<!-- GENERAL INFO -->
<TABLE>
  <TR><TD align="right">
    <j:choose>
      <j:when test="${build.result=='SUCCESS'}">
        <IMG SRC="${rooturl}static/e59dfe28/images/32x32/blue.gif" />
      </j:when>
          <j:when test="${build.result=='FAILURE'}">
        <IMG SRC="${rooturl}static/e59dfe28/images/32x32/red.gif" />
      </j:when>
      <j:otherwise>
        <IMG SRC="${rooturl}static/e59dfe28/images/32x32/yellow.gif" />
      </j:otherwise>
    </j:choose>
  </TD><TD valign="center"><B style="font-size: 200%;">BUILD ${build.result}</B></TD></TR>
  <TR><TD>Build URL</TD><TD><A href="${rooturl}${build.url}">${rooturl}${build.url}</A></TD></TR>
  <TR><TD>Project:</TD><TD>${project.name}</TD></TR>
  <TR><TD>Date of build:</TD><TD>${it.timestampString}</TD></TR>
  <TR><TD>Build duration:</TD><TD>${build.durationString}</TD></TR>
  <TR><TD>Test duration:</TD><TD>${testDuration}</TD></TR>
</TABLE>
<BR/>
<!-- JUnit TEMPLATE  hudson.tasks.junit.TestResult   -->
<% def junitResultList = it.JUnitTestResult
try {
 def cucumberTestResultAction = it.getAction("org.jenkinsci.plugins.cucumber.jsontestsupport.CucumberTestResultAction")
 junitResultList.add(cucumberTestResultAction.getResult())
} catch(e) {
        //cucumberTestResultAction not exist in this build
}
// API: http://hudson-ci.org/javadoc/hudson/tasks/junit/PackageResult.html
%>
<!-- JUnit TEMPLATE: all tests PASS FAIL SKIP >
<% 
if (junitResultList.size() > 0) { %>
 <TABLE width="100%">
 <TR><TD class="bg1" colspan="2"><B>${junitResultList.first().displayName}</B></TD></TR>
 <% junitResultList.each{
  junitResult -> %>
     <% junitResult.getChildren().each { packageResult -> %>
        <TR><TD class="bg2" colspan="2"> <B>TEST SUITE: ${packageResult.getName()} Failed: ${packageResult.getFailCount()} test(s), Passed: ${packageResult.getPassCount()} test(s)</B>, Skipped: ${packageResult.getSkipCount()} test(s), Total: ${packageResult.getPassCount()+packageResult.getFailCount()+packageResult.getSkipCount()} test(s)</TD></TR>
        <% packageResult.getChildren().each{ suite -> 
               suite.getChildren().each{ test ->
           def colour = "lightgreen"
           def highlight1=""
           def highlight2=""
           RESULT = test.getStatus() // FAILED or PASSED or SKIPPED
           if (RESULT == hudson.tasks.junit.CaseResult.Status.FAILED || RESULT == hudson.tasks.junit.CaseResult.Status.REGRESSION) {
               colour = "#ffcccc" 
               highlight1="<B>"
               highlight2="</B>"
           }
           if (RESULT == hudson.tasks.junit.CaseResult.Status.SKIPPED) { colour = "#ffffb3" }
         %>
          <TR bgcolor="${colour}"><TD class="test" colspan="2">${highlight1}<li>${RESULT}: ${test.getFullName()} </li>${highlight2}</TD></TR>
        <% } }
      }
 } %>
 </TABLE>
 <BR/>
<%
} %>
end of test.groovy
</body>
</html>
e.g. output (text only no colours/formatting)
start test.groovy 
TEST RESULT: 18 total, 18 pass, 0 fail, 0 skip. 
Workspace : /var/lib/jenkins/jobs/jobname-1/workspace 
Project Name : jobname-1 #20
BUILD SUCCESS 
Build URL   http://jenkinsurl:port/job/jobname-1/20/
Project:    jobname-1 
Date of build:  Mon, 23 Jan 2017 09:29:00 +0000 
Build duration: 10 min 
Test duration:  267.12
Test Results 
TEST SUITE: suitename1 Failed: 0 test(s), Passed: 3 test(s), Skipped: 0 test(s), Total: 3 test(s) 
 * PASSED: suitename1.testclass.testname1
 * PASSED: suitename1.testclass.testname2
 * PASSED: suitename1.testclass.testname3
TEST SUITE: suitename2 Failed: 2 test(s), Passed: 1 test(s), Skipped: 0 test(s), Total: 3 test(s) 
 * PASSED: suitename2.testclass.testname1
 * FAILED: suitename2.testclass.testname2
 * REGRESSION: suitename2.testclass.testname3
end of test.groovy