Fix flaky legacy pipelines spec

What does this MR do and why?

The test was failing frequently because the Ajax request to play the manual job was running after the wait_for_requests, so the backend state of the job remained at scheduled instead of pending.

To reduce the chance of this happening, click the pipeline status graph and look for a pending icon. Checking the state of the backend shouldn't be required, but we include it just in case.

Closes #367483 (closed)

How to set up and validate locally

Run this:

bundle exec rspec ./spec/features/projects/pipelines/legacy_pipelines_spec.rb

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