workflows: add job to aggregate all test results

This makes it possible to define a stably-named required status check.

.github/workflows/ci.yaml

@@ -7,7 +7,22 @@ on:
     branches: [ master ]
 
 jobs:
-  build-and-test:
+  # This job aggregates all matrix results and is used for a GitHub required status check.
+  test_results:
+    if: ${{ always() }}
+    runs-on: ubuntu-latest
+    name: Test Results
+    needs: [build_and_test]
+    steps:
+      - run: |
+          result="${{ needs.build_and_test.result }}"
+          if [[ $result == "success" || $result == "skipped" ]]; then
+            exit 0
+          else
+            exit 1
+          fi
+
+  build_and_test:
     runs-on: ubuntu-latest
     strategy:
       matrix: