fake-editor: replace expectpath with dump-path

[mlcui-corp]

This allows us to assert the expected path in the test itself, rather than in the fake editor.

(This will be used for when we de-UNC-ify invocations of the editor, see #3986 for more details.)

Checklist

If applicable:

• [ ] I have updated CHANGELOG.md
• [ ] I have updated the documentation (README.md, docs/, demos/)
• [ ] I have updated the config schema (cli/src/config-schema.json)
• [ ] I have added tests to cover my changes

[mlcui-corp]

expectpath is now not used anywhere - should we remove it in this PR?

[mlcui-corp]

Resolved comments - PTAL.