Reuse test commands in cover target

... to maintain the commands in a single place.

Change-Id: I3da4278bac6c8038805c6034ccd83a4509af7eb7
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

tox.ini

@@ -22,12 +22,12 @@ commands =
 [testenv:cover]
 deps =
   {[testenv]deps}
-  coverage>=5.0
+  coverage>=5.0 # Apache-2.0
 setenv =
   PYTHON=coverage run --source taskflow --parallel-mode
 commands =
   coverage erase
-  stestr run {posargs}
+  {[testenv]commands}
   coverage combine
   coverage html -d cover
   coverage xml -o cover/coverage.xml