E2E test: update source editor preview e2e test

What does this MR do and why?

Resolves #411583 (closed), this test is failing consistently since !119029 (merged)

As per !119029 (merged) I have stopped enabling the feature flag source_editor_toolbar as the toolbar is the same with or without this flag enabled.

Also data-qa-selectors were removed but there were still methods using them. I had to create a new method for showing the preview.

How to set up and validate locally

bundle exec rspec qa/specs/features/browser_ui/3_create/source_editor/source_editor_toolbar_spec.rb

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