Remove cctest's ability to run multiple tests (which has long been
deprecated and mostly broken). We can then make platform & V8
initialisation be part of running the test's Run method.

In particular, this allows us to inject custom logic into the platform
initialisation, like setting up a platform wrapper. Add a
TEST_WITH_PLATFORM which exercises this by registering a platform
factory on the test, and wrapping the default platform using this
factory. This allows these tests to guarantee that the lifetime of the
platform is longer than the lifetime of the isolate.

As a result of this, we can also remove the complexity around draining
platform state in the TestPlatform (since it will now have a longer
lifetime than the Isolate using it), and as a drive-by clean up the
TestPlaform to use a CcTest-global "default platform" instead of trying
to scope over the "current" platform.

As another drive-by, change the linked-list of CcTests and the linear
search through it into an std::map of tests.

Change-Id: I610f6312fe042f29f45cc4dfba311e4184bc7759
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3569223Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79772}