[embedder] Consistent naming for GL/Metal tests (flutter/engine#38141)

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 https://github.com/flutter/engine/pull/38133

Related: https://github.com/flutter/cocoon/pull/2340
Issue: https://github.com/flutter/flutter/issues/116381

engine/src/flutter/shell/platform/embedder/BUILD.gn

@@ -317,11 +317,11 @@ if (enable_unittests) {
     deps = [ ":embedder_unittests_library" ]
 
     if (test_enable_gl) {
-      sources += [ "tests/embedder_unittests_gl.cc" ]
+      sources += [ "tests/embedder_gl_unittests.cc" ]
     }
 
     if (test_enable_metal) {
-      sources += [ "tests/embedder_unittests_metal.mm" ]
+      sources += [ "tests/embedder_metal_unittests.mm" ]
     }
   }