Can't configure TestCase

[fornellas]

TestSlide has a hacky support for TestCase.assert* methods. While this works, it prevents users from tweaking things such as maxDiff.

The proposed solution to fix this is:

• Drop the hacky self._test_case.
• Make usage of the TestCase integration, perhaps by adding doing self.add_test_case() at Context.__init__, and assigning it to self.test_case.
• Keep the __getattr__ logic, but forward it to self.test_case.

With that, users should be able to do self.test_case.maxDiff = new_value.

[lucastosetto]

Hello! Do you still need help with that? I'd like to contribute.