ci(tests): Also test macOS-latest

Author: tony

.github/workflows/tests.yml

@@ -18,6 +18,13 @@ jobs:
             tmux-version: '2.6'
           - python-version: '3.7'
             tmux-version: 'master'
+          - python-version: '3.7'
+            tmux-version: '2.6'
+            os: macOS-latest
+          - python-version: '3.7'
+            tmux-version: 'master'
+            os: macOS-latest
+
     steps:
     - uses: actions/checkout@v1