Just compute the MD5 of the advertising identifier and use it as your test device id - Source
Here is apple's documentation to get the advertising identifier. Check section Getting the Advertising Identifier
The ASIdentifierManager class provides apps with access to an
  identifier that can be used only for serving advertisements, as well
  as a flag which indicates whether a user has limited ad tracking.
This class is expected to be used by developers who are implementing a
  system for serving ads. These organizations typically provide app
  developers with the code that interacts with this class. App
  developers may need to link against the Ad Support framework to use
  that code, but they should not need to interact with the
  ASIdentifierManager class directly.
To get the advertising identifier:
- Get a reference to the shared instance of this class using the
  sharedManager method. 
- Check whether advertising tracking is limited using the
  advertisingTrackingEnabled property. 
If the user has limited ad tracking, use the advertising identifier
  only for the following purposes: frequency capping, attribution,
  conversion events, estimating the number of unique users, advertising
  fraud detection, and debugging.
- Get the advertising identifier using the advertisingIdentifier
  property.