tests: move pytest settings from kallithea/tests/pytest.ini to setup.cfg

pytest.ini was added in 9b8ba0f1c87b but didn't work. pytest only uses one
configuration file and does not merge settings from multiple files [1]. The
first detected file is setup.cfg thus should contain all relevant
configuration.

[1] http://pytest.org/latest/customize.html#initialization-determining-rootdir-and-inifile