add backcompat job that I forgot before (#4862)

.yamato/training-backcompat-tests.yml

+
+test_mac_backcompat_2020.1:
+  {% capture editor_version %}2020.1{% endcapture %}
+  {% capture csharp_backcompat_version %}1.0.0{% endcapture %}
+  # This test has to run on mac because it requires the custom build of tensorflow without AVX
+  # Test against 2020.1 because 2020.2 has to run against package version 1.2.0
+  name: Test Mac Backcompat Training {{ editor_version }}
+  agent:
+    type: Unity::VM::osx
+    image: ml-agents/ml-agents-bokken-mac:0.1.4-492264
+    flavor: b1.small
+  variables:
+    UNITY_VERSION: {{ editor_version }}
+  commands:
+    - |
+      python3 -m venv venv && source venv/bin/activate
+      python -m pip install pyyaml --index-url https://artifactory.prd.it.unity3d.com/artifactory/api/pypi/pypi/simple
+      python -m pip install unity-downloader-cli --index-url https://artifactory.prd.it.unity3d.com/artifactory/api/pypi/pypi/simple --upgrade
+      unity-downloader-cli -u {{ editor_version }} -c editor --wait --fast
+      # Backwards-compatibility tests.
+      # If we make a breaking change to the communication protocol, these will need
+      # to be disabled until the next release.
+      python -u -m ml-agents.tests.yamato.standalone_build_tests --build-target=mac
+      python -u -m ml-agents.tests.yamato.training_int_tests --csharp {{ csharp_backcompat_version }}
+    - |
+      python3 -m venv venv_old && source venv_old/bin/activate
+      python -m pip install pyyaml --index-url https://artifactory.prd.it.unity3d.com/artifactory/api/pypi/pypi/simple
+      python -u -m ml-agents.tests.yamato.training_int_tests --python 0.16.0
+  triggers:
+    cancel_old_ci: true
+    recurring:
+      - branch: master
+        frequency: daily
+  artifacts:
+    logs:
+      paths:
+        - "artifacts/standalone_build.txt"
+        - "artifacts/inference.nn.txt"
+        - "artifacts/inference.onnx.txt"
+        - "artifacts/*.log"
+    standalonebuild:
+      paths:
+        - "artifacts/testPlayer*/**"
+        - "artifacts/models/**"