tests: register mark in pytest configuration

tests: register mark in pytest configuration

Unregister marks generates warnings like:

PytestUnknownMarkWarning: Unknown pytest.mark.docker - is this a typo?
You can register custom marks to avoid this warning

https://docs.pytest.org/en/latest/mark.html

Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>
(cherry picked from commit ac4f8763aab8ad62771b412662230349676242c7)