Add performance tests to tox.ini

Without this, we are reusing the default env.
This is a problem, as it does extra stuff we don't need, like
fetching containers. To represent a user behaviour, no prefetch
should be done.

tox.ini

@@ -29,6 +29,11 @@ commands =
 commands =
     pytest -n auto test_unit.py --junitxml={toxinidir}/junit_unit.xml []
 
+[testenv:performance]
+commands =
+    # No need to fetch containers or run things in parallel
+    pytest -n0 -m "serial" test_{envname}.py --junitxml={toxinidir}/junit_performance.xml []
+
 [testenv:fetch-all]
 commands =
     m8a-fetch-all-containers