Improve test case coverage for java.util.Objects

[Ao-senXiong]

API in java.util.Objects are heavily used in industry such as Objects.requireNonNull() and we don't have test case that use it yet.

[wmdietl]

What do you want to test about it?
Tests for a particular type system?
We do not have test cases for every annotation that is in eisop/jdk.

[Ao-senXiong]

I would like add some tests for nullness checker that use the API in jdk.

[wmdietl]

Sure, make a PR if you feel that particular method is worth testing. But again, trying to achieve coverage of all annotations in eisop/jdk seems futile work.

Also note that we have the permit-nullness-assertion-exception.astub stub file that changes the signature of that method.
So, you would need to test with and without that stub file.