Fixed macos kafka CI (#1807)

* Update tox to run kafka separately.

* Included clarification comments.

tox.ini

@@ -20,7 +20,11 @@ commands_pre =
     poetry install --sync --no-root
 commands =
     python -m pytest {posargs} -n auto \
-        {toxinidir}/tests
+        {toxinidir}/tests --ignore={toxinidir}/tests/kafka
+    # kafka tests are failinig for macos when running in parallel
+    # with the entire test suite. So, we run them separately.
+    python -m pytest {posargs} -n auto \
+        {toxinidir}/tests/kafka
 set_env =
     GITHUB_SERVER_URL = {env:GITHUB_SERVER_URL:https\://github.com}
     GITHUB_REPOSITORY = {env:GITHUB_REPOSITORY:SeldonIO/MLServer}
@@ -41,7 +45,12 @@ commands =
         {toxinidir}/runtimes/mllib \
         {toxinidir}/runtimes/lightgbm \
         {toxinidir}/runtimes/mlflow \
-        {toxinidir}/runtimes/huggingface
+        {toxinidir}/runtimes/huggingface \
+        --ignore={toxinidir}/tests/kafka
+    # kafka tests are failinig for macos when running in parallel
+    # with the entire test suite. So, we run them separately.
+    python -m pytest {posargs} -n auto \
+        {toxinidir}/tests/kafka
 set_env =
     GITHUB_SERVER_URL = {env:GITHUB_SERVER_URL:https\://github.com}
     GITHUB_REPOSITORY = {env:GITHUB_REPOSITORY:SeldonIO/MLServer}