ci: 🎡 add test step for entire monorepo in CI

.github/workflows/monorepo-validate.yaml

@@ -23,3 +23,14 @@ jobs:
           node-version: '12.x'
       - run: yarn install
       - run: yarn audit
+
+  test:
+    name: Test
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-node@v1
+        with:
+          node-version: '12.x'
+      - run: yarn install
+      - run: yarn test