Cover all aliases with at least one test

[psychoslave]

As the project evolved, it introduced terms which are not yet covered by tests.

Concerned terms include:

• [x] __id__
• [x] __send__
• [ ] SystemCallError::===
• [ ] instance_eval
• [ ] instance_exec
• [ ] method_missing
• [ ] singleton_method_added
• [ ] singleton_method_removed
• [ ] singleton_method_undefined
• [x] equal?
• [x] eql?