CI: Make test report available as artifact (#1484)

Make the HTML test report available as an GitHub Actions artifact.
One ZIP files for download, available for 3 days.

Backport of #1290.

.github/workflows/ci.yml

@@ -74,3 +74,9 @@ jobs:
       run: .github/workflows/test_simple.sh
     - name: Run tests
       run: .github/workflows/test_thorough.sh
+    - name: Make HTML test report available
+      uses: actions/upload-artifact@v2
+      with:
+        name: testreport-${{ matrix.os }}
+        path: testreport
+        retention-days: 3