pytest.ini: s/repo_test/repo_tests/

otherwise pytest complains at seeing the marker of repo_tests, like

============================================================================================================== warnings summary ==============================================================================================================
tests/test_scylla_download.py:9
  /home/kefu/dev/scylla-ccm/tests/test_scylla_download.py:9: PytestUnknownMarkWarning: Unknown pytest.mark.repo_tests - is this a typo?  You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/stable/how-to/mark.html
    @pytest.mark.repo_tests

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html

this warning is distracting.

scylladb / scylla-ccm

pytest.ini: s/repo_test/repo_tests/ #529