[6.0] Implement a version of `SwiftSyntaxMacrosTestsSupport` that is framework agnostic

[ahoppen]

• Explanation: SwiftSyntaxMacrosTestsSupport does not depend on XCTest or Foundation can be used to write macro tests using swift-testing.
• Scope: SwiftSyntaxMacrosTestSupport
• Risk: Moderate, mostly shuffles code around modules and adds new API
• Testing: Verified manually that SwiftSyntaxMacrosGenericTestsSupport can be used to test macros using swift-testing
• Issue: rdar://119519713
• Reviewer: @rintaro @bnbarham @hamishknight on https://github.com/apple/swift-syntax/pull/2647

[ahoppen]

@swift-ci Please test