Add a 10-minute timeout to tests

The default is six hours for the whole job.

We've been seeing PyTest freezing during some macOS runs.  Not sure
what's up, but at least we can not wait for a six hour timeout.

Most tests seem to take between two and three minutes, so this seems
like quite a generous timeout.

Author: jholveck

.github/workflows/tests.yml

@@ -107,9 +107,11 @@ jobs:
         run: python -m pip install tensorflow
 
       - name: Tests (GNU/Linux)
+        timeout-minutes: 10
         if: matrix.os.name == 'linux'
         run: xvfb-run python -m pytest
       - name: Tests (macOS, Windows)
+        timeout-minutes: 10
         if: matrix.os.name != 'linux'
         run: python -m pytest