gqltest: add a test to cover repository with space in its name

Created by: unknwon

Example "Other" external service config:

{
  "url": "https://<username>:<password>@sourcegraph.visualstudio.com/sourcegraph/_git/",
  "repositoryPathPattern": "sourcegraph.visualstudio.com/{repo}",
  "repos": [
    "Test%20Repo"
  ]
}

Follow up of #15745