Revert "Allow to have no parallel tests in pytest"

This reverts commit 68cce96.
There is no need to do this, instead we should describe our own
tox env.

tox.ini

@@ -22,7 +22,7 @@ commands =
     m8a-fetch-language-containers {envname}
     # -n auto tells pytest to use as many processors as detected
     # to run all the tests from the module test_{envname} in parallel
-    pytest -n auto -m "not serial" --suppress-no-test-exit-code test_{envname}.py --junitxml={toxinidir}/junit_parallel.xml []
+    pytest -n auto -m "not serial" test_{envname}.py --junitxml={toxinidir}/junit_parallel.xml []
     pytest -n0 -m "serial" --suppress-no-test-exit-code test_{envname}.py --junitxml={toxinidir}/junit_serial.xml []
 
 [testenv:unit]