chore: switch to proper cache file now we have it, i.e. package-lock.json

as per https://github.com/actions/setup-node#caching-global-packages-data

.github/workflows/tests.yml

@@ -106,7 +106,7 @@ jobs:
       with:
         node-version: '12'
         cache: 'npm'
-        cache-dependency-path: 'js/package.json'
+        cache-dependency-path: 'js/package-lock.json'
         
     - name: Setup Mocha
       run: npm install -g mocha