t2028: fix minor error and issues in newly-added "worktree move" tests

t2028: fix minor error and issues in newly-added "worktree move" tests

Recently-added "git worktree move" tests include a minor error and a few
small issues. Specifically:

 * checking non-existence of wrong file ("source" instead of
   "destination")

 * unneeded redirect (">empty")

 * unused variable ("toplevel")

 * restoring a worktree location by means of a separate test somewhat
   distant from the test which moved it rather than using
   test_when_finished() to restore it in a self-contained fashion

 * having git command on the left-hand-side of a pipe ("git foo | grep")

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Acked-by: Duy Nguyen <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>