``AioHTTPTestCase`` is more async friendly now.

For people who use unittest and are used to use :py:exc:`~unittest.TestCase`
it will be easier to write new test cases like the sync version of the :py:exc:`~unittest.TestCase` class,
without using the decorator `@unittest_run_loop`, just `async def test_*`.
The only difference is that for the people using python3.7 and below a new dependency is needed, it is ``asynctestcase``.
