E2E test: don't wait for fork MR to load

What does this MR do and why?

Resolves #415967 & #420275 (closed)

To avoid this bug occurring during the fork MR tests, we wait for ensure the fork-icon element is present on the page, rather than waiting for the entire page to load before proceeding.

How to set up and validate locally

QA_LOG_LEVEL=DEBUG QA_GITLAB_URL=http://192.168.4.31:3000  GITLAB_QA_PASSWORD_1=XXX bundle exec rspec qa/specs/features/browser_ui/3_create/merge_request/merge_merge_request_from_fork_spec.rb

Can also see in the logs that there is no waiting for loading to complete... between these messages. This indicates that we are no longer waiting for the page to load.

[Aug 29 2023 07:36:12 UTC (QA Tests)] WARN  -- Potentially Slow Code 'has_element? fork-icon' took 1.262s
[Aug 29 2023 07:36:12 UTC (QA Tests)] DEBUG -- has_element? :fork-icon (wait: 10) returned: true

MR acceptance checklist

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