Some tests were calling set_fake_editor to ensure they had a sane no-op
editor set. Now that all the editor setting is done in subshells these
tests can rely on EDITOR=: and so do not need to call set_fake_editor.
Also add a test at the end to detect any future additions messing with
the exported value of $EDITOR.
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk> Signed-off-by: Junio C Hamano <gitster@pobox.com>