t3404: remove uneeded calls to set_fake_editor

t3404: remove uneeded calls to set_fake_editor

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>