Today, because we set the host_cpu to x64, unittests don't get built for e.g. host_debug_unopt_arm64 on macOS.

If you override the gn arguments (enable_unittests = true), the tests build fine. On my machine, they mostly seem to run fine as well. However, in a few attempts on CI, I saw some failures.

https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket/8783970492851834417/+/u/test:_Host_Tests_for_host_debug_unopt_arm64/stdout saw a failure in FlutterMutatorViewTest.TransformedFrameIsCorrect.

EmbedderTest.CompositorMustBeAbleToRenderKnownSceneWithSoftwareCompositor also fails due to golden byte differences: https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket/8783974394687752593/+/u/test:_Host_Tests_for_host_debug_unopt_arm64/stdout

The golden test likely either needs to be skipped on one platform or have an updated golden byte (or, even better, use SkiaGold and make sure that the platform details are added in such a way that we end up with a golden for each platform).

/cc @jmagman @cyanglaz @cbracken