Allow parameterization of flags in tests.

This change adds a custom JUnit runner that allows running tests with
various feature flag configurations.

Tests can be annotated with `@ParameterizeSyntheticTargets` to test
against synthetic targets like STAGING, PROD, and MAINLINE.

Bug: 438342600
Flag: EXEMPT changing tests
Test: Changing tests
Change-Id: I80b5652c9108d903009fdafa1dffffa7ca30c7c7