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/test.yaml

@@ -22,5 +22,8 @@ jobs:
         name: Install dependencies
         run: npm ci
       -
-        name: Run tests
+        name: Run unit tests
         run: npm run test:unit
+      -
+        name: Run integration tests
+        run: npm run test:integration