Use stestr for testing

The CI is currently blocked because of using pytest directly.
This patch unblocks the CI.

Change-Id: I7f5b84e5100aef7f8453f6b6f613bf03468074b3

tox.ini

@@ -14,9 +14,8 @@ passenv =
 	PROMETHEUS_*
 	OBSERVABILITY_*
 deps = .[test]
-       pytest
 
-commands = pytest {posargs} {env:OS_TEST_PATH}
+commands = stestr run --slowest {posargs} --test-path {env:OS_TEST_PATH}
 
 [testenv:pep8]
 basepython = python3
@@ -44,7 +43,7 @@ deps = .[test]
        -r{toxinidir}/test-requirements.txt
 commands =
        bash tools/fix_ca_bundle.sh
-       pytest {posargs} {env:OS_TEST_PATH}
+       stestr run --slowest {posargs} --test-path {env:OS_TEST_PATH}
 
 
 [flake8]