gqltestutil: always return external service ID after creation

Created by: unknwon

We've hit 3x empty external service ID since yesterday's attempt to ignore transient repo-updater error:

• https://buildkite.com/sourcegraph/sourcegraph/builds/72443#f7a403c6-0222-4320-8f50-fd1d32912dfa
• https://buildkite.com/sourcegraph/sourcegraph/builds/72483#41b2d46d-d732-467d-b2f7-49c349c728e4
• https://buildkite.com/sourcegraph/sourcegraph/builds/72471#83d46bbb-b4aa-476b-bd43-5fa103344ab0

It turns out that the external service ID is not collected correctly, so fixing it.