CI with GitHub Actions: bump the timeout for front-end unit tests (#17682)

With the code being instrumented for code coverage, and another
additional time to analyze + send the code coverage to codecov.io,
the typical execution runs awfully close to the 10-min limit.
Hence, add some extra room.

.github/workflows/frontend.yml

@@ -78,7 +78,7 @@ jobs:
 
   fe-tests-unit:
     runs-on: ubuntu-20.04
-    timeout-minutes: 10
+    timeout-minutes: 12
     steps:
     - uses: actions/checkout@v2
     - name: Prepare Node.js