Migrate more end-to-end test suites to Swift Testing

[bkhouri]

Migrate four ArgumentParserEndToEndTests suites from XCTest to
Swift Testing:

• ShortNameEndToEndTests
• JoinedEndToEndTests
• LongNameWithSingleDashEndToEndTests
• SimpleEndToEndTests

Each file previously imported both Testing and XCTest from earlier
prep work but still used XCTestCase and XCTAssert*. Convert to
@Suite struct + @Test func, AssertParse → expectParse,
XCTAssertEqual → #expect(a == b), and XCTAssertThrowsError →
#expect(throws: (any Error).self). No new helpers required.

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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