Add E2E Test for editor md preview panel side by side

What does this MR do and why?

Adds E2E coverage for the new feature introduced #280798 (closed). A new button !87145 (merged) allows to easily give the live-preview of markdown right away.

The feature is currently behind a feature flag ":source_editor_toolbar".

Test case link here: https://gitlab.com/gitlab-org/gitlab/-/quality/test_cases/367749

Screenshots or screen recordings

How to set up and validate locally

Run: bundle exec bin/qa Test::Instance::All <local_ip> -- qa/specs/features/browser_ui/3_create/web_ide/source_editor_toolbar_spec.rb

MR acceptance checklist

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