tests: use pytest instead of unittest

pytest provides session scoped fixtures, making it much easier
to reuse resources across tests.

tox.ini

@@ -33,8 +33,7 @@ setenv =
     LD_LIBRARY_PATH={[vars]vddk_path}
     LD_PRELOAD=
 commands =
-    stestr run --slowest --concurrency 1 \
-               --test-path tests/integration/ {posargs}
+    python -m pytest -v tests/integration/ {posargs}
 
 [testenv:integration-coverage]
 description = Integration tests with coverage report.
@@ -44,11 +43,10 @@ setenv =
     {[testenv:integration]setenv}
 deps =
     {[testenv]deps}
-    pytest
     pytest-cov
     pycobertura
 commands =
-    pytest --cov=openvixdisklib --cov-report=term-missing \
+    python -m pytest --cov=openvixdisklib --cov-report=term-missing \
            --cov-report=xml -p no:cacheprovider tests/integration/ {posargs}
     pycobertura show --format html --source openvixdisklib \
         coverage.xml -o coverage.html
@@ -73,6 +71,7 @@ commands =
 description = Type checks (mypy).
 deps =
     mypy
+    pytest
 commands =
     mypy {[vars]all_path}
 
@@ -89,6 +88,3 @@ ignore = E125,E251,W503,W504,E305,E731,E117,W605,F632
 exclude = .venv,.git,.tox,dist,build,*.egg
 # Same length as ruff.
 max-line-length = 88
-
-[stestr]
-test_path = ./tests