Add option for test discovery info mode

Add a command line option to enable test discovery info mode, where
enabling it will cause test discovery to run and report results but not
actually affect the build. This is useful for rolling out to new targets
without immediately enabling test discovery mode.

Test: atest build_test_suites_test
Bug: 388833029
Change-Id: I8baf73e6353e4c3d7d6836c4f9823341a7f8b5dd