Test: Add coverage to Web IDE spec for Workflow extension

Issue: #478037 (closed)

What does this MR do and why?

In this MR we improve the feature spec coverage for the VSCode-based Web IDE. This coverage would have caught some recent issues that failed in downstream QA.

Notes from commit:

• Had to disable marketplace FF because the feature specs can't load gitlab-static.net.
• This would have prevented some recent QA failures. gitlab-com/gl-infra/production#18371 (closed)

MR acceptance checklist

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Related to #478037 (closed)