Rename GITLAB_TOKEN to GITLAB_TEST_TOKEN in test utilities

Aayushrequested to merge
aayushsaini/gitlab-runner:issue-39159 into main

This MR introduces GITLAB_TEST_TOKEN for integration tests and removes GITLAB_TOKEN. 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.

Related: #39159 (closed)

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