Fix mock backend address in dev environment

src/environments/environment.dev.ts

@@ -4,7 +4,7 @@
 
 export const environment = {
   production: false,
-  testcenterUrl: '/api/',
+  testcenterUrl: 'http://localhost:8000/',
   appPublisher: 'IQB - Institut zur Qualitätsentwicklung im Bildungswesen',
   apiVersionExpected: '5.0.1'
 };