Implement warning for unmatched test specs

docs/release-notes.md

@@ -173,6 +173,7 @@ new design.
   * When deferred tu runtime, it behaves like `CHECK`, and not like `REQUIRE`.
 * You can have multiple tests with the same name, as long as other parts of the test identity differ (#1915, #1999, #2175)
   * Test identity includes test's name, test's tags and and test's class name if applicable.
+* Added new warning, `UnmatchedTestSpec`, to error on test specs with no matching tests
 
 
 ### Fixes