[PLAT-5956] Ensure intended copy of Mac test fixture is run

[nickdowell]

Goal

Prevent the wrong app bundle being run during E2E testing.

Changeset

The macOS test fixture now accepts a command line argument -register which will cause it to register itself with Launch Services, and (crucially) unregister all other copies of the app using the private _LSUnregisterURL API.

env.rb no longer moves the test fixture into /Applications - it will unzip if needed and register the app if found in the standard location of {repo}/features/fixtures/macos/output.

The fixture now prints its path in the log, to aid debugging in the future.

To run tests locally against a fixture built via Xcode, simply ensure that features/fixtures/macos/output is empty or missing.

Testing

Verified locally and via CI.

[github-actions]

Infer: No issues found 🎉

OCLint: No issues found 🎉

Bugsnag.framework binary size did not change - 1,253,528 bytes

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[twometresteve]

When I run this locally, the initial launch of the app stays up and I have to close it manually for the scenarios to start in anger.

However, everything seems to be ok on CI (I triggered a separate full run), so if you're happy then I'm happy.