Migrate large end-to-end suites to Swift Testing

[bkhouri]

Migrate seven ArgumentParserEndToEndTests suites from XCTest to Swift
Testing:

• SourceCompatEndToEndTests
• UnparsedValuesEndToEndTests
• DefaultSubcommandEndToEndTests
• FlagsEndToEndTests
• NestedCommandEndToEndTests
• RepeatingEndToEndTests
• RepeatingEndToEndTests+ParsingStrategy (extension of the above)

FlagsEndToEndTests previously had a small FlagsEndToEndTestsSWT
Swift Testing suite alongside the XCTest class; fold its
parsingCaseIterable_Help test back into the main suite so all
flag-parsing tests live in one place.

NestedCommandEndToEndTests had a private AssertParseFooCommand
helper that curried AssertParseCommand with Foo.self as the root;
port it to expectParseFooCommand calling expectParseCommand for
the same purpose.

DefaultSubcommandEndToEndTests previously used
AssertParseCommandErrorMessage (XCTest-only, not ported to Swift
Testing yet). Inline the parse-and-check-message logic in that one
test with #require on Test.current semantics.

RepeatingEndToEndTests+ParsingStrategy is an extension of
RepeatingEndToEndTests and must be migrated in the same PR so
extension RepeatingEndToEndTests refers to the new @Suite struct
rather than the old XCTestCase class.

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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