starboard: roll up the remaining unittest targets

[yell0wd0g]

This PR continues the work of #4831 and rolls up all other targets I could find that looked like unit tests into starboard_unittests.

It also gets the changes of #5031, so that the aspect of starboard/BUILD.gn is closer to the final state.

With this I see

[==========] 2106 tests from 57 test suites ran. (535744 ms total)
[  PASSED  ] 1818 tests.
[  SKIPPED ] 288 tests, listed below:

  YOU HAVE 1 DISABLED TEST

(the disabled test was from before, the skipped ones are new, see further below).

Other topics:

• media_util_test.cc needed operator== on SbMediaColorMetadatas, so I added it as extern.
• VideoDecoderTests/...beneath_the_canopy_137_avc... were failing because PlayerComponentsFactory couldn't load libopenh264.so. My debian did not have that link (only libopenh264.so.7), when I added it it started working. Something to watch out for in bots.
• Similarly PlayerComponentsTest needed the libfdk-aac pacakge, with a vanilla link that I had to do by hand.
• E-AC-3 and AC-03 related audio tests refer to Dolby Vision, for which I could not find an opensource library (plugin for ffmpeg); this CL adds code to SKIP them on Linux bots.
• //starboard/raspi/shared/:starboard_platform_tests target was empty, this CL removes it.

Bug: b/384819454