Migrate more end-to-end suites to Swift Testing

[bkhouri]

Migrate six ArgumentParserEndToEndTests suites from XCTest to Swift
Testing:

• TransformEndToEndTests
• OptionGroupEndToEndTests
• ValidationEndToEndTests
• OptionalEndToEndTests
• PositionalEndToEndTests
• SubcommandEndToEndTests

OptionGroupEndToEndTests and SubcommandEndToEndTests previously
relied on XCTestExpectation via the
TestableParsableArguments/TestableParsableCommand protocols. Add
Swift Testing counterparts to TestHelpers+SwiftTesting.swift:

• TestExpectation: a reference-typed flag that carries a fulfilled
  bool. Since ArgumentParser's parse/validate/run pipeline is
  synchronous, we don't need a timeout-based expectation like
  XCTestExpectation.
• TestableSwiftTestingParsableArguments /
  TestableSwiftTestingParsableCommand: mirror the XCTest protocols;
  their default validate() / run() fulfill didValidateExpectation
  / didRunExpectation, which tests then assert with
  #expect(command.didValidateExpectation.fulfilled).

Centralizing the type means future changes (e.g. moving to
confirmation, adding a counter, adding Sendable) touch only the
helper.

ValidationEndToEndTests's FooCommand.run() contained an inline
XCTAssertEqual; convert it to #expect(foo == bar) so the assertion
routes through Swift Testing when run under @Test.
SubcommandEndToEndTests's Math.run() gets the same treatment.

PositionalEndToEndTests keeps
disabled_parsing_BadlyFormedPositional as an untagged method
(matching the previous "not discovered" behavior of the disabled_
prefix under XCTest); its XCTFail becomes Issue.record.

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]

I'm sure how to migrate this test to Swift Testing.

@natecook1000 @rauhul: Any ideas here?

[natecook1000]

I'm not sure what exactly this is trying to test! I think we could just manually update a value in the validate() method and check that it worked. What do you think?

[bkhouri]

The test is using XCTestExpection in SubSubCommand, which is indirectly a subcommand of BaseCommand. See right line 215.

I'm not familiar with XCTestExpectation.