Add validation check that @ClassRule can only return an implementation of TestRule

[npathai]

Presently @ClassRule can be an implementation of TestRule or MethodRule but it does not make sense to allow MethodRule. So as per discussion on #1016 with @kcooney creating this issue.

// should fail validation as @ClassRule should only allow implementation of `TestRule` but passes
    public class ClassRuleTest {
        @ClassRule
        public static MethodRule methodRule() {
            return new MethodRule() {

                @Override
                public Statement apply(Statement base, FrameworkMethod method, Object target) {
                    return base;
                }
            };
        }

        @Test
        public void doNothing() {

        }
    }

[npathai]

@kcooney I am working on it. Adding separate validation in CLASS_RULE_VALIDATOR and CLASS_RULE_VALIDATOR to only allow TestRule.