E2E test: Fix source DDL selector

What does this MR do and why?

Adds Qa Selector to source branch dropdown for new merge request (compare branch) page.

On the new-MR-compare-branches page the source branch drop down list qa-selector is missing. This selector isn't used for all test cases that involving creating an MR, only when the below banner with the Create MR button does not show, the E2E tests use this page.

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 (comment 1278763117)

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, so this flow isn't excercised all of the time.

When this button doesn't appear the tests will navigate to the new-mr-compare-branches page. This is where the updated selector is.

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

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

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