Support NonisolatedNonsendingByDefault

### Motivation If you enabled the upcoming feature flag `NonisolatedNonsendingByDefault` in your package and use Swift Testing with a Suite Trait, you'll probably hit upon the issue that `provideScope(for:testCase:performing:)` doesn't conform to the protocol, even though from the error message they're the same. The fix is to annotate the closure with `@concurrent` but this should be done in the framework ### Proposed solution Support `NonisolatedNonsendingByDefault` by default ### Alternatives considered Make users fix it, a workaround but not ideal ### Additional information _No response_