UTS checker and renderer are completely untested

[jackfirth]

No test constructs UTS-annotated syntax, so check-universal-tagged-syntax and check-source-universal-tagged-syntax in private/universal-tagged-syntax.rkt (60.5% coverage) and syntax-replacement-render-using-uts in private/syntax-replacement.rkt never run — about 95 uncovered lines combined. Testing them means hand-building syntax objects with uts-content and uts-separators properties.

This might be worth deferring until #586 settles the UTS spec rather than pinning tests to a moving prototype, but filing it so the gap is tracked.

Part of #680.