E2E tests of the project-level merge method options

We have plenty of E2E tests covering the default merge commit merge method, but only one (browser_ui/3_create/merge_request/rebase_merge_request_spec.rb) that uses the Fast-forward merge option, and no E2E tests that involve the Merge commit with semi-linear history option.

We should add tests to cover each merge method for each of:

• Merge
• Squash and merge
• Rebase and merge