Fix flaky content_editor_spec

What does this MR do?

Tweaks use_new_editor method for content_editor_spec to wait for the new editor container to appear before attempting to add content. It should help to avoid flakiness seen in #338553 (closed).

NB If the standard Capybara wait of a few seconds is not going to be enough and we see more failures, this will be reported in #332629 (closed) (the new editor is still in beta and some issues can be expected).

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

BROWSER_HEADLESS=false QA_DEBUG=true  bundle exec bin/qa Test::Instance::All http://localhost:3000 -- qa/specs/features/browser_ui/3_create/wiki/content_editor_spec.rb

Does this MR meet the acceptance criteria?

Conformity

• I have self-reviewed this MR per code review guidelines.
• I have followed the style guides.