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.

.eslintrc

@@ -52,6 +52,12 @@ overrides:
     rules:
       import/no-unresolved: [0]
       import/no-extraneous-dependencies: [0]
+  - files: ["*.test.js"]
+    env:
+      jest: true
+  - files: ["*.config.js"]
+    rules:
+      import/no-unused-modules: [0]
 
 rules:
   accessor-pairs: [2]