Fix logic for waiting on CI YAML content

What does this MR do and why?

Fixes a race condition where we're clicking "Validate" before the editor is populated with the YAML contents.

We can see in the failure screenshot that we're getting the Please provide content of .gitlab-ci.yml error.

Screenshots or screen recordings

How to set up and validate locally

Run bin/rspec spec/features/projects/ci/lint_spec.rb.

Specs here fail consistently for me locally before this change.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #370204 (closed)