iOS,macOS: Build/test test_metal_surface_unittests

[cbracken]

Years ago, @chinmaygarde created TestMetalSurface and wrote a unit test for it in ed30d77.

And some things that should not have been forgotten were lost. History became legend. Legend became myth. And for four and a half years, the test passed out of all knowledge. Until, when chance came, it ensnared a new bearer. The test came to the creature @cbracken, who took it deep into the tunnels of the GN build files. And there it consumed him. The test brought to @cbracken unnatural long life. For 500 hours, it poisoned his mind. And in the gloom of @cbracken's cave, it waited. Darkness crept back into the forests of the world. Rumour grew of a shadow in the East - whispers of a nameless fear. And the Test of Unit perceived its time had now come.

Issue: flutter/flutter#158998

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I did not list at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

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