tests/test-locale.t

revert: use `set_untracked` instead of `drop` when applicable

This is the new shiny API.

Differential Revision: https://phab.mercurial-scm.org/D11026

  $ LANG=nonexistent LC_ALL=nonexistent LANGUAGE=nonexistent hg version -q
  Mercurial Distributed SCM (version *) (glob)