`SwiftSyntaxMacrosTestSupport` should warn when being used from Swift Testing

### Description `SwiftSyntaxMacrosTestSupport` does not work with Swift Testing and will cause false positives when running tests. The library should detect when it's being invoked in a Swift Testing environment and fail to build or at the very least show a warning that's hard to miss. This is a stop gap until #2720 is resolved ### Steps to Reproduce _No response_