Add apex_test.skip_validations

Run validations for apex_test modules as well. If a test apex needs to
be invalid for testing, it needs to specify which validation it needs to
skip. This is to prevent cases like using apex_test to avoid a certain
validation, but accidentally skipping all validations.

For now, host_apex_verifier/apex_sepolicy_tests can be skipped.

Bug: 363090062
Test: m nothing --no-skip-soong-tests
Test: mmma system/apex
Change-Id: I2d14f25f579eabe92ed2bd0e28b5ed19583df99d