Add tests for checking displayed current branch in the pipeline editor

What does this MR do?

For #332961 (comment 625314200):

We could utilize FE feature tests to test that selecting an option in the dropdown results in seeing the text of that option on the page. I think this https://gitlab.com/gitlab-org/gitlab/-/blob/master/spec/features/projects/branches/user_views_branches_spec.rb can be a good example.

This MR adds simple feature tests to check that the current branch is displayed correctly on the branch switcher.

Screenshots or Screencasts

No UI changes, just tests.

Does this MR meet the acceptance criteria?

Conformity

• [-] I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
• [-] I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)