Instruct pytest to pre-load the cov plugin early (#1791)

This is necessary because in some cases there's a race condition
with xdist and loading `pytest_cov` early mitigates that.

Author: webknjaz

tox.ini

@@ -32,6 +32,7 @@ commands =
   {envpython} -m pytest \
   --junitxml "{toxworkdir}/junit.{envname}.xml" \
   {posargs:\
+    -p pytest_cov \
     --cov ansiblelint \
     --cov "{envsitepackagesdir}/ansiblelint" \
     --cov-report term-missing:skip-covered \