Related to #7879
This commit updates `zig build test` to run Xcode tests, too. These run
in parallel to the Zig tests, so they don't add any time to the test.
The Xcode tests will _not_ run when: (1) the target is not macOS, or (2)
the `-Dtest-filter` option is non-empty. This makes it so that this
change doesn't affect non-macOS and doesn't affect the general dev cycle
because you usually will run `-Dtest-filter` when developing a core
feature.
I didn't add a step to only run Xcode tests because I find that when I'm
working in Xcode I'm probably going to run the tests from there anyways.
The integration with `zig build test` is just a convenience, especially
around CI.
Speaking of CI, this change also makes it so this will run in CI.
