x/build: allow x/mobile tests run on darwin/arm64

[changkun]

The x/mobile is an experimental repository that skips many builders. Its current builders include android/386, android/amd64, and ios/arm64.

While working on CL 346150, we found that tests in x/mobile are not tested on a macOS. This may cause issues:

• cmd/gomobile may be partially broken without any notice (Mac tests had broken for quite a long time until recent fixes), which leads to the runtime support for iOS being unverifiable (not a wanted behavior).
• it can cause the review process to confuse because a Mac-specific change is also not verifiable through a Trybot run.

Since we already have a darwin/arm64 environment (darwin-arm64-11_0-toothrot), we may use it as a builder for x/mobile.

A further concern is that x/mobile tests may require an Xcode installation with a valid Apple Developer account. However, Xcode is not forward-compatible (earlier Xcode versions may not work with later Xcode-specific changes). If a CL targets a newer Xcode release, it may not be testable until the environment is ready. Consider this reason, a dedicated environment with multiple Xcode installations may be better.

@dmitshur @hyangah @hajimehoshi

[cherrymui]

cc @golang/release

[ydnar]

A further concern is that x/mobile tests may require an Xcode installation with a valid Apple Developer account. However, Xcode is not forward-compatible (earlier Xcode versions may not work with later Xcode-specific changes). If a CL targets a newer Xcode release, it may not be testable until the environment is ready. Consider this reason, a dedicated environment with multiple Xcode installations may be better.

Most of the tests in x/mobile can be run without a valid Apple Developer account.

I’d be happy to submit a CL that skips certain tests if an Apple Developer account can’t be located on the build host.