Rewrite `abbr` function

The usage is still the same, but it's a lot more robust, and also no
longer assumes $fish_user_abbreviations must be a universal variable.

This also fixes the unexpected error output when calling `abbr -a` with
no existing abbreviations.

Calling `abbr -a` with an abbreviation that already exists now silently
overwrites the abbreviation, just like `function` and `bind` do, instead
of complaining.
Fix test breakage from 14724401f

We can't use $PATHS to test the :-splitting because the global config
file adds extra paths based on /etc/paths and /etc/paths.d.

Ideally fish would have a way to suppress behavior like that, but for
the time being it doesn't.