[embedder] Consistent naming for GL/Metal tests

While I've sent a patch to mark the shell/platform/embedder/tests and
fixtures directories as test exempt (since they are tests), by
convention, tests should end in _unittests.* for C++ tests, and _test.*
for Dart tests. This renames for consistency with other tests such as
embedder_a11y_unittests.cc.

Uncovered by #38133

Related: flutter/cocoon#2340
Issue: flutter/flutter#116381

Author: cbracken