[ci] Enforce Swift 6 strict concurrency during testing

[jmagman]

Enforce Swift 6 strict concurrency during testing to catch thread checker and other main actor errors and regressions as we migrate package tests to Swift 6 flutter/flutter#150388.

See https://www.swift.org/documentation/concurrency.

Follow-up from discussion here: #10762 (comment)

Note if you don't use @MainActor there will be a main thread checker error because the test won't run on the main thread:

PointerInterceptorView.swift:12 UIView.initWithFrame: must be used from main thread only

Though the test still passed. I'm not sure how to make it an error instead of a warning, without converting the whole thing to a test plan (which seems overkill)

Confirmed in d7d4ea7#diff-2941dc3894ffd50d9031c12999cf0ec618d817525423934f46442f797a1b86fdL10 that forcing a main actor error does indeed fail the test:

Testing failed:
	Main actor-isolated property 'backgroundColor' can not be referenced from a nonisolated context
	Testing cancelled because the build failed.

Part of flutter/flutter#180755

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[jmagman]

Oh right, we tested this already and quick_actions_ios has a violation flutter/flutter#150388 (comment)

https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket/8692640612516226529/+/u/Run_package_tests/native_test/stdout