circleci: shrink python tracebacks

Mostly we're not testing python code, so verbose python tracebacks are
unhelpful. Add --tb=short to the pytest args to cut down on the noise.

To override this during testing, set the "extra_params" parameter on the
circleci job to "--tb=auto" or "--tb=long".

.circleci/config.yml

@@ -154,7 +154,7 @@ jobs:
             if [ -n "$TEST_FILTER" ]; then
               TEST_FILTER="-k $TEST_FILTER"
             fi
-            pytest $TEST_FILE $TEST_FILTER $EXTRA_PARAMS
+            pytest --tb=short $TEST_FILE $TEST_FILTER $EXTRA_PARAMS
       - run:
           # CircleCI artifacts are preserved one file at a time, so skipping
           # this step isn't a good idea. If you want to extract the