Override Test ID to ensure trait dependencies are cached

[jflan-dd]

I ran into an issue where reference typed dependencies weren't correctly cached if they were updated in-place rather than being fully overridden.

The root cause of this seems to be that Test.current isn't set during prepare(for test: Test) and as a result the CacheKey.testIdentifier was nil during dependency setup.

This PR creates an internal TestID struct that can be explicitly overridden during dependency prep, but perhaps there's a more elegant solution that could be implemented in xctest-dymaic-overlay directly.

This PR is dependent on pointfreeco/swift-issue-reporting#137

[jflan-dd]

It seems like this could also be solved with a TestExecuting conformance if https://forums.swift.org/t/pitch-custom-test-execution-traits/75055/14 lands, since Test.current is set before _executeTraits is called

Is there a plan for how to be backwards compatible with different versions of swift-testing as new features are added?

[stephencelis]

@jflan-dd Thanks for looking into this and putting together a PR! We'll try to review it soon.

[jflan-dd]

@stephencelis I created a separate PR in swift-issue-reporting to support overriding the TestContext which greatly simplified this PR