tests: restrict pytest test collection to kallithea/tests

When the kallithea root directory contains a populated virtualenv, pytest
would also collect tests in python packages installed there.

Restrict the tests to be considered to any test_*.py file inside
kallithea/tests.

Additionally, by renaming unwanted test files in kallithea/tests/scripts to
_not_ match this pattern, we can completely get rid of the 'norecursedirs'
option.