style: run tests via npm script insread of custom command

I.e. not call mocha directly but let the script defined in package.json do it's job.

.github/workflows/tests.yml

@@ -116,6 +116,6 @@ jobs:
       working-directory: js
       
     - name: Run unit tests
-      run: mocha
+      run: npm test
       working-directory: js