Run the tests as part of the PR checks

Now that we finally have _something_ in the way of a test suite, let's
run it.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

.github/workflows/pr.yml

@@ -10,4 +10,6 @@ jobs:
       - name: npm install
         run: npm install
       - name: run ESLint
-        run: npm run lint
+        run: npm run lint
+      - name: run tests
+        run: npm run test