Ignore flakey plugins.GitLabPluginTest.testGitLabMultibranchPipeline

[timja]

see #1461

Testing done

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4. Link to relevant issues in GitHub or Jira
5. Link to relevant pull requests, esp. upstream and downstream changes
6. Ensure you have provided tests - that demonstrates feature works or fixes the issue

[timja]

amends #1368, cc @car-roll