Fix coverage test

File .testr.conf was removed in patch [1]. It will make the
coverage ci job failed (and tox -ecover also).

This patch will change cover command in tox.ini to fix coverage
test without bring .testr.conf back.

[1] https://review.openstack.org/#/c/504342/

Change-Id: Id9635a42255272396c1a5f45105360325fc74d2e
Closes-Bug: #1719083

.testr.conf

@@ -1,4 +0,0 @@
-[DEFAULT]
-test_command=OS_STDOUT_CAPTURE=1 OS_STDERR_CAPTURE=1 OS_LOG_CAPTURE=1 ${PYTHON:-python} -m subunit.run discover -t ./ ${OS_TEST_PATH:-./kuryr_libnetwork/tests/unit} $LISTOPT $IDOPTION | cat
-test_id_option=--load-list $IDFILE
-test_list_option=--list

tox.ini

@@ -42,10 +42,15 @@ commands = flake8
 commands = {posargs}
 
 [testenv:cover]
+setenv =
+    VIRTUAL_ENV={envdir}
+    PYTHON=coverage run --source kuryr_libnetwork --parallel-mode
 commands =
-  python setup.py test --coverage --testr-args='{posargs}' \
-    --coverage-package-name=kuryr_libnetwork
-  coverage report
+    stestr run {posargs}
+    coverage combine
+    coverage html -d cover
+    coverage xml -o cover/coverage.xml
+    coverage report
 
 [testenv:docs]
 commands = python setup.py build_sphinx