tests: add as little code as possible in __init__.py kallithea/tests/__init__.py contained quite a lot of code, including the test base class TestController. This in itself may be considered bad practice. Specifically, this poses a problem when using pytest 3.0+, in which asserts in some files are not automatically rewritten to give improved assert output. That problem can be fixed by explicitly registering such files for assertion rewriting, but that register call should be executed _before_ said files are imported. I.e. if the register call is in kallithea/tests/__init__.py, assert calls in __init__.py itself can not be rewritten. Since the TestController base class does effectively contain asserts, and we do not want to execute the register call from somewhere outside the kallithea/tests directory, we need to move the TestController class to another file (kallithea/tests/base.py) so we can have a register call in __init__.py before loading base.py. While not strictly necessary to fix the mentioned pytest problem, we take the opportunity to fully clean __init__.py and move everything to the new kallithea/tests/base.py. While doing so, unnecessary imports are removed, and imports are ordered alphabetically. Explicit imports of symbols from modules that were already imported as a whole, are removed in favor of fully qualifying the references (e.g. tempfile._RandomNameSequence).