Follow-up from "Add missing handler for protected branches Deploy key"

The following discussion from !149863 (merged) should be addressed:

• @vyaklushin started a discussion: (+1 comment)

  Note I believe the test can be improved. 🤔 If have any ideas how, feel free to update it.

Refactor spec/frontend/protected_branches/protected_branch_edit_spec.js:

• create a helper function for repeated steps needed to interact with dropdown (see for example describe('preselected item') block)
• see if there is a way to limit the number of times create() is called