install pytest-xdist and run with (hopefully) more processes in CI (#3178)

* install pytest-xdist and run with (hopefully) more processes in CI

* try -n 2

.circleci/config.yml

             . venv/bin/activate
             mkdir test-reports
             pip freeze > test-reports/pip_versions.txt
-            pytest --cov=ml-agents --cov=ml-agents-envs --cov=gym-unity --cov-report html --junitxml=test-reports/junit.xml -p no:warnings
+            pytest -n 2 --cov=ml-agents --cov=ml-agents-envs --cov=gym-unity --cov-report html --junitxml=test-reports/junit.xml -p no:warnings
 
       - run:
           name: Verify there are no hidden/missing metafiles.

test_requirements.txt

 # Test-only dependencies should go here, not in setup.py
 pytest>4.0.0,<6.0.0
 pytest-cov==2.6.1
+pytest-xdist