Move direct superclass checking to `test/t/superclass.rb`.
Test all ISO defined classes direct superclass except `Object` class.
Move mrbgems ISO direct superclass test to `superclass.rb`.
Skips test if class isn't defined.
Close #2332.
Merge pull request #2396 from take-cheeze/superclass_check

Move direct superclass checking to `test/t/superclass.rb`.