GitLab UI: 1029-add-borderless-tertiary-loading-button-variant integration test

What does this MR do?

Integration test for updates to the loading button. From gitlab-ui!1808 (comment 434684908):

A question I have is whether or not removing the orange will impact the product, or if the existing orange loaders will just fall back to the new dark default?

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

N/A

Security

N/A

Related to #1029 (closed)