Remove ref_path from coverage CSV path

What does this MR do?

Related to #294032 (closed)

We no longer use ref_path in the path to download the CSV for test coverage. We were making ref_path equal to an empty string because of how the frontend was adding the query params. By utilizing mergeUrlParams, we can get rid of the empty string and make our path more reliable.

Changelog not needed since this is developer facing only.

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

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done