`SwiftSyntaxMacrosTestSupport` should warn when being used from Swift Testing

[0xTim]

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