Supressing GC in .NET for Unit Test performance

Question

I'm writing a rather convoluted unit test that uses Parallel.ForEach on a list of one million ints.

Using JetBrains DotTrace, I noticed that a lot of garbage collection is going on in the code under test, it's taking up 10% of the total test running time and about 14% of the time spent in that method.

Here's a screenshot of the profiler:

The culprit, I guess, is a local variable that captures the current time. Here is the code:

public PatientQueryMessage CreateQueryMessage(string patientNumber)
{
    var nowOffset = DateTimeOffset.Now; // GC nightmare?

    return new PatientQueryMessage
    {
        MessageHeader = new MessageHeader
        {
            DateTimeMessage = new TimeStamp(nowOffset),
            MessageControlId = $"{patientNumber}{nowOffset.Ticks.ToString().Substring(7, 5)}"
            // More properties set, but filtered out for brevity
        },
        QueryDefinition = new QueryDefinition
        {
            QueryDateTime = new TimeStamp(nowOffset),
            QueryId = $"{patientNumber}{nowOffset:MMddHHmmssf}"
            // More properties set, but filtered out for brevity
        }
    };
}

Now, here are the important bits of the test code:

Parallel.ForEach(myNumberArray,
    new ParallelOptions { MaxDegreeOfParallelism = maxThreads },
    () => /* TLocal Init */,
    (number, loopState, localResult) =>
    {
        var queryMessage = _queryMessageHelper.CreateQueryMessage(patientNumber);

        // Brevity...

        return localResult;
    },
    resultList =>
    {
         // Brevity...
    }
);

In the loop body, is there any way I can temporarily shut down or otherwise restrain the Garbage Collector, so that it will only cleanup once everything has settled down?

Note that I care a lot more about the time it takes for the test to run than how much memory it consumes.