[CI] Run unit tests for Android implementation on CI

[danielgomezrico]

We already have tests for the android implementation but they are not been run on CI for each PR

Folders and tests

So we have:

• lib/: library implementation, the interace that clients use.
  • tests/: test for the flutter side of the library
• android/
  • android/src/test: these are the tests we want to run

What we have tried

• Run flutter plugin tests, running flutter test at the root of the project and it runs test/iterable_flutter_test.dart but not android ones.

• We have seen these docs flutter/wiki/Plugin-Tests, but I still don't figure out how to run them.

[danielgomezrico]

I opened a question around this on stackoverflow:
https://stackoverflow.com/questions/69407522/flutter-how-to-run-unit-tests-over-a-plugin-native-code-for-android