Cherry pick PR #5127: starboard: roll up the remaining unittest targets - #5413

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

With this I see
```
[==========] 1454 tests from 55 test suites ran. (8612 ms total)
[  PASSED  ] 989 tests.
[  SKIPPED ] 465 tests, listed below:
...
  YOU HAVE 1 DISABLED TEST
```

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

Media-related topics:

- A number of tetst (e.g. `AudioDecoderTest` and others) needed a number
of system libraries to support codecs like AAC, AC3 etc but those are
part of non-free Debian repos, so couldn't install those in Docker. This
CL adds a new method `IsProprietaryAudioCodec()` to be used to SKIP test
cases related to those codecs, when the build flag
`SKIP_PROPRIETARY_CODEC_TESTS` is present.
- `VideoDecoderTests` and `PlayerComponentsTest` failed flakily due to
X11 use not being torn down correctly; they need `xvfb-run`. This makes
me think those are not pure unit-tests but integration tests like
`nplb`. For the time being they are commented out with a TODO.
- `media_util_test.cc` needed `operator==` on `SbMediaColorMetadata`s,
so I added it as extern.
- `FileCacheReader` needs to find the media files under the output
folder's `content/` directory. Files are under the output folder's
`starboard/content`, because of the starboard toolchain. This CL leaves
a quick hack and a TODO() to find a proper solution.

Finally:
- `//starboard/raspi/shared/:starboard_platform_tests` target was empty,
this CL removes it.

Bug: b/384819454
(cherry picked from commit 82430e3)