Add another way for TestingModule to get its output files

In OutputFiles method, TestingModule is able to get its own output
files by reading its module base property. If the TestingModule
never updates its outputFiles property, it will fall back to use the OutputFileProducer interface.

Only empty string tag case is added in this CL since all the testing modules are getting the output files using empty string tag.

Test: CI
Bug: 339477385
Change-Id: I3009ae45d8d909653e3b6b222bced9ccabbaede7