Skip visual test in master branch

What does this MR do?

Skip the gitlab:ui:visual test on the master branch as I'm concerned if the following occurred:

1. Legitimate CSS change in application.css that affects gitlab-ui visual test.
2. [skip visual] is used in the commit to enable green pipeline
3. Red pipeline in master until someone updates gitlab-ui snapshots (this delay could block a lot of work in CE/EE master)

Ideally, we would want it to trigger the pipeline automatically, but we can do that after the code freeze. There is still value in having this test even if it's not run in master

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/58517

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer