Test coverage for clearing mod changes

[mnholtz]

Per discussion following https://github.com/pixiebrix/pixiebrix-extension/pull/9240 (see slack thread here) we'd like to add e2e and/or unit test coverage where appropriate covering clearing mod changes. Includes coverage for the following flows:

• Modifying mod metadata/name/options & clearing those changes
• Modifying mod component in some way & clearing changes
• Deleting mod component & clearing changes