GH Actions: actually run tests against Windows

This was already correctly done for the `quicktest` and the `coverage` jobs, but apparently, changing the `runs-on` was missed for the `test` job.

Oops.

Fixed now.

Author: jrfnl

.github/workflows/test.yml

@@ -45,7 +45,7 @@ jobs:
       group: ${{ github.workflow }}-${{ github.job }}-${{ strategy.job-index }}-${{ github.ref }}
       cancel-in-progress: true
 
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
     needs: build
 
     strategy: