E2E test: use search_and_select for compare branch ddl

What does this MR do and why?

On the new-MR-compare-branches page the source branch drop down list has been updated and the tests need to be updated. When the below banner with the the Create MR button does not show, the E2E tests use this page to create a new MR.

The page url is something like http://192.168.4.31:3000/gitlab-qa-sandbox-group-6/qa-test-2023-02-10-15-44-59-2cde3f1106a6b964/project-30c131ddfaf4dcf3/-/merge_requests/new (not to be confused with the compare branches page)

Resolves #391249 (closed) #391271 (closed) #391263 (closed)

How to set up and validate locally

Usually when creating an MR the tests click the following button which jumps to the new MR page.

QA_LOG_LEVEL=DEBUG QA_GITLAB_URL=http://192.168.4.31:3000 bundle exec rspec qa/specs/features/browser_ui/3_create/merge_request/create_merge_request_spec.rb However, when this button doesn't appear (not sure when this occurs) the tests will navigate to the compare branches page.

So to verify this fix locally, we must put a breakpoint here and then manually close the above popup to exercise this flow.

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.