Migrate tooling test targets to Swift Testing

[bkhouri]

Migrate the two tooling test targets from XCTest to Swift Testing:

• Tests/ArgumentParserGenerateDoccReferenceTests/GenerateDoccReferenceTests
• Tests/ArgumentParserGenerateManualTests/GenerateManualTests

Add two Swift Testing helpers to TestHelpers+SwiftTesting.swift
that mirror the XCTest-only versions in TestHelpers.swift:

• expectGenerateManual(multiPage:command:) — runs the
  generate-manual executable and compares against .mdoc snapshots.
• expectGeneratedReference(command:doccFlavored:) — runs the
  generate-docc-reference executable and compares against .md
  snapshots (docc-flavored or plain).

Both preserve the #function-based snapshot lookup so existing
baselines under Snapshots/ are found without renaming.

With this PR, no test files in the repository import XCTest.

Closes #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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