pkg/edit: Make tests work when run as root.
pkg/edit: Make tests work when run as root.

The default prompt is different for root users, so when run as root, a lot of
tests will fail because the prompt looks different. This commit sets up an
edit:prompt that emulates the default prompt for non-root users as part of the
test fixture.

elves/elvish,elves/elvish,elves/elvish,elves/elvish,elves/elvish