I've scoured the internet for an answer to this question and got here. It would seem that no, there is no way to create an universal androidTest APK from individual feature modules' instrumentation tests.
I'll probably look into creating an instrumentation test only library that depends on my app- and feature modules. https://developer.android.com/studio/test#use_separate_test_modules_for_instrumented_tests
If I'm not mistaken, that should enable me to build a single test-APK that runs all the tests against the universal APK.
^That didn't work due to some manifest issues:
> Failed to calculate the value of task ':instrumentation:processVariantDebugManifest' property 'testedApplicationId'.
> Collection has more than one element.
I guess we're stuck with running the tests module by module...