code: public static String generateTokenString(String jsonResName, Set<InvalidClaims> invalidClaims) throws Exception {
          return generateTokenString(jsonResName, invalidClaims, null);
      }
docs: Utility method to generate a JWT string from a JSON resource file that is signed by the privateKey.pem
  test resource key, possibly with invalid fields.
  
  @param jsonResName - name of test resources file
  @param invalidClaims - the set of claims that should be added with invalid values to test failure modes
  @return the JWT string
  @throws Exception on parse failure