Preserve pre-install functionality, at least for langpacks

[Rob--W]

Firefox is currently working on the ability to allow users to remove sideloaded extensions: https://blog.mozilla.org/addons/2019/10/31/firefox-to-discontinue-sideloaded-extensions/

This may affect the usability of the --pre-install flag. This feature is currently documented (in web-ext run --help) to be used for older versions of Firefox, but the underlying technique would also be needed to test langpacks ( #1313 ).

So we should make sure that the functionality works, at least for langpacks.