build: make rtc_test depend on test_main by default

and replace a couple of test-only rtc_executable targets with rtc_test.
benchmarks, which links against a separate main, now needs to be an executable instead of a test.

Bug: webrtc:467294026
Change-Id: Idf2d76db691d91d68531cc9719bc0e0480b6723e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/432801
Commit-Queue: Jeremy Leconte <jleconte@google.com>
Reviewed-by: Philipp Hancke <philipp.hancke@googlemail.com>
Reviewed-by: Jeremy Leconte <jleconte@google.com>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46568}