Migrate more test suites to Swift Testing

[bkhouri]

Migrate seven test suites from XCTest to Swift Testing:

• InputOriginTests (converted to a parameterized test using
  CustomTestStringConvertible for descriptive case IDs)
• SendableTests (compile-time-only suite)
• RawRepresentableEndToEndTests
• EqualsEndToEndTests
• EnumEndToEndTests
• SingleValueParsingStrategyTests
• AsyncCommandEndToEndTests (marked @Suite(.serialized) with
  @MainActor tests because the suite shares the global statusCheck
  state that XCTest previously serialized implicitly)

Relates to #710

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

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[bkhouri]

This is ready to review, but I'm marking as a draft to merge the stack PRs all in a single go.