Fix a JS race in a spec

What does this MR do?

Fixes intermittent spec failures in spec/features/projects/settings/user_changes_default_branch_spec.rb by waiting for JS that sets the project default branch to finish executing before we set our own value

What are the relevant issue numbers?

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

Closes #56860 (closed)

label ~Create ~backstage test