Consider not using GITLAB_TOKEN for integration tests

The following discussion from !5982 (merged) should be addressed:

• @timofurrer started a discussion:

  I'm a little bit hesitant to use the GITLAB_TOKEN variable for this. I've seen "oupsies" in the past. It's a variable that's used with many toolings and people often have (too) high-privileged tokens in their shell exported as GITLAB_TOKEN.

  If there is no strong reason to use GITLAB_TOKEN, I'd recommend using GITLAB_TEST_TOKEN (to follow the naming pattern of the username variable) instead. WDYT?