add initial integration tests

Integration tests are executed using vue-cli-service with double quotes as following: `vue-cli-service test:unit "tests/integration/**/*.spec.ts"`. Using single quotes (mochajs/mocha#1828) works on macOS and Ubuntu but does not on Windows (tests are not found). Double quotes is the only portable way that works on all three platforms (mochajs/mocha#3136).

.github/workflows/deploy-site.yaml

@@ -90,7 +90,9 @@ jobs:
         working-directory: site
       -
         name: "App: Run tests"
-        run: npm run test:unit
+        run: |
+          npm run test:unit
+          npm run test:integration
         working-directory: site
       - 
         name: "App: Build"