CheckReturnValue: Ignore in TestNG

The `try/execute/fail/catch` and expected exception patterns are ignored for junit. Please add testng to the [known libraries](https://github.com/google/error-prone/blob/9f50f708894f8aba387e7cfaa0a2987af90ccfd1/core/src/test/java/com/google/errorprone/bugpatterns/checkreturnvalue/CheckReturnValueWellKnownLibrariesTest.java). See [usage](https://github.com/ben-manes/caffeine/commit/b4c92e4275308c23ce537e2b9b8c36feed15f98d#diff-94b1a51ddeade35bcef75c9af9b43009f0d60177e87fc681f6743d2cf0848d6c) that required suppressing.