feat: warn when SwiftSyntaxMacrosTestSupport is used with Swift Testing (#3190)

[DPrakashhh]

Description:
This PR addresses #3190 by adding a runtime safety guard to SwiftSyntaxMacrosTestSupport. This is a stop-gap measure until full Swift Testing support (#2720) is implemented.

The Problem
SwiftSyntaxMacrosTestSupport is XCTest-based. When used with the new Swift Testing framework, assertions may fail silently or produce false positives, leading to developer confusion.

The Solution
I have added a thread-safe, once-per-process runtime check in _SwiftTestingDetection.swift.

Key Features:

• Smart Detection: Identifies Swift Testing via environment variables (SWIFT_TESTING_) and command-line arguments.
• XCTest Safety: Automatically detects standard XCTest environments to prevent false warnings for existing tests.
• Swift 6 Ready: Uses NSLock and nonisolated(unsafe) to satisfy strict concurrency requirements.
• High Visibility: Prints a distinct ASCII banner to stderr that is "hard to miss."
• Configuration: - SWIFTSYNTAX_TESTSUPPORT_SILENCE=1: Suppresses the warning.
  • SWIFTSYNTAX_TESTSUPPORT_STRICT=1: Triggers a preconditionFailure for strict enforcement.

Verification

• Regression Testing: Verified with the full test suite (3,470 tests passed).

[DPrakashhh]

@ahoppen
Sir, kindly check this PR, I have opened this few time ago :)

[ahoppen]

I’ll leave this PR to @rintaro and @hamishknight to review who have more context here at the moment.