test(spec): pin the unknown-key diagnostic to one text

The two runtimes each claimed to raise the other's message and nothing
checked it. Both now render the committed text for the committed key.

pkg/spec/src/web-runtime.ts

@@ -694,6 +694,7 @@ export const __testing__ = {
   selectorFromString,
   selectorFromObject,
   SELECTOR_KEYS,
+  unknownSelectorKeyMessage,
   xpathStringLiteral,
 };