chore(ci): add test conclusion step

We require the test matrix to succeed before a PR can be merged.
Unfortunately, the rule is rather complex as it has to test that each
element of the test matrix completed successfully.

By adding another step which runs only of the matrix succeeded, we can
check that this ran, as opposed to each element of the matrix.

Signed-off-by: JP-Ellis <josh@jpellis.me>

.github/workflows/test.yml

@@ -60,6 +60,16 @@ jobs:
         with:
           token: ${{ secrets.CODECOV_TOKEN }}
 
+  test-conlusion:
+    name: Test matrix complete
+
+    runs-on: ubuntu-latest
+    needs:
+      - test
+
+    steps:
+      - run: echo "Test matrix completed successfully."
+
   example:
     name: Example