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.