Cover the new file page in the single file editor specs

What does this MR do and why?

Companion to Preview new files before committing them (!253963 - merged), as part of Add preview for new markup files (#15501). Test coverage only, no implementation changes.

The single file editor specs were written almost entirely against the edit page fixture (a #file_path input). With Preview new files before committing them (!253963 - merged) giving the new file page (a #file_name input) its Write/Preview tabs, sort each spec onto the fixture of the page where its scenario actually plays out:

• New file page: scenarios that start from an empty file name
  • loading the Markdown extensions (Unify the definitions of "Markdown" behind edit... (!253025 - merged)), and races while typing (Fix races between Markdown extension loading an... (!251306 - merged))
  • falling back from the live preview (Follow mid-edit renames to Markdown in the "Pre... (!250889 - merged))
  • the preview request (Follow mid-edit renames in the preview renderer (!251790 - merged)): newly covers that the #file_name value is submitted as file_path (the fixture includes the tabs Preview new files before committing them (!253963 - merged) adds)
• Edit page: scenarios that rename a pre-filled file name
  • the install / uninstall specs from Unify the definitions of "Markdown" behind edit... (!253025 - merged) shared most of their setup, so they are merged into one spec each covering the full lifecycle

Along the way:

• fold the preview error specs into the preview request describe (the success and failure sides of the same POST)
• align the snippet editor's Markdown extension specs (Sync snippet Markdown live preview with mid-edi... (!252609 - merged)) with naming a new, unnamed file

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