Potential fix for flaky spec/features/jira_connect/branches_spec.rb

What does this MR do and why?

The previous test may have been racy because the test attempted to enter my-source-branch in the Search box, but selecting the master for the Source branch in the browser may cause the Search box to be overwritten with master. To prevent this race, we wait for the search request to complete before filling out another term.

Relates to #385660 (closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.