Prevent lingering object from test_config.

Using the config_tmdpir fixture across all tests in this module caused
a lingering LineParser to make test_debug fail.
I still don't know why, but scoping the config_tmpdir fixture to only
the test class that was creating ~/.config/qute_test fixes the issue,
and still prevents creation of a user tempdir.

tests/unit/config/test_config.py

@@ -35,8 +35,6 @@ from qutebrowser.config.parsers import keyconf
 from qutebrowser.commands import runners
 from qutebrowser.utils import objreg, standarddir
 
-pytestmark = pytest.mark.usefixtures('config_tmpdir')
-
 
 class TestConfigParser:
 
@@ -297,6 +295,7 @@ class TestKeyConfigParser:
         assert new == new_expected
 
 
+@pytest.mark.usefixtures('config_tmpdir')
 @pytest.mark.integration
 class TestDefaultConfig: