E2E test: remove unnecessary login mr from fork test

What does this MR do and why?

Fixes #409194 (closed)

The merge_merge_request_from_fork_spec.rb E2E test has been recently de-quarantined and it has been occasionally failing when switching users immediately after creating an MR. There seems to be requests still running when the sign out button is clicked which means the token isn't cleared and then we get a 422, this seems to only occur due the speed of the actions of the automated test.

This MR waits for requests to complete before signing out. (Similar fix has been done here and here this fix greatly reduced the number of failures, but occasionally it still can occur)

It was also noticed that the test was logging out and in with the same user, so now we check that if the user is currently signed in, don't sign in again.

How to set up and validate locally

bundle exec rspec qa/specs/features/browser_ui/3_create/merge_request/suggestions/merge_merge_request_from_fork_spec.rb

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