Add frontend testing, require node 12 (#15315)

- Add basic frontend unit testing infrastructure using jest in ESM mode
- Rename 'make test' to 'make test-backend'
- Introduce 'make test-frontend' and 'make test' that runs both
- Bump Node.js requirement to v12. v10 will be EOL in less than a month.
- Convert all build-related JS files to ESM.

I opted to run frontend tests run as part of the compliance pipeline because
they complete fast and are not platform-specific like the golang tests.

.drone.yml

@@ -70,11 +70,17 @@ steps:
       - make checks-backend
     depends_on: [lint-backend]
 
+  - name: test-frontend
+    image: node:14
+    commands:
+      - make test-frontend
+    depends_on: [lint-frontend]
+
   - name: build-frontend
     image: node:14
     commands:
       - make frontend
-    depends_on: [lint-frontend]
+    depends_on: [test-frontend]
 
   - name: build-backend-no-gcc
     pull: always