pytest: log timestamps (#221)

The CI job duration increased significantly after switching to
self-hosted runners.

We'll update the pytest invocation to log timestamps, which
should help us uncover potential bottlenecks.

test/performance/tox.ini

@@ -37,6 +37,8 @@ commands =
         --maxfail 1 \
         --tb native \
         --log-cli-level DEBUG \
+        --log-format "%(asctime)s %(levelname)s %(message)s" \
+        --log-date-format "%Y-%m-%d %H:%M:%S" \
         --disable-warnings \
         {posargs} \
         {toxinidir}/tests