Fixes a CI failure

In one of the API tests, `gon.relative_url_root` was set to
`http://host.invalid` which is not a relative root. This breaks the
tests.

ee/changelogs/unreleased/11315-jest-failure-on-master.yml

+---
+title: Fixes a CI failure in jest
+merge_request: 11586
+author:
+type: fixed

ee/spec/frontend/api_spec.js

@@ -4,7 +4,7 @@ import Api from 'ee/api';
 
 describe('Api', () => {
   const dummyApiVersion = 'v3000';
-  const dummyUrlRoot = 'http://host.invalid';
+  const dummyUrlRoot = '/gitlab';
   const dummyGon = {
     api_version: dummyApiVersion,
     relative_url_root: dummyUrlRoot,