test: Unset ALTERNATE_EDITOR before running emacsclient
ALTERNATE_EDITOR causes emacsclient to run an alternate editor if the
emacs server is not ready. This can collide with intended
functionality in test-lib.sh.
If the ALTERNATE_EDITOR is set but empty, emacsclient runs emacs
daemon and tries to connect to it. When this happens the emacs run by
test-lib.sh fails to start the server and the subsequent attempts to
use the server fail because the daemon started by emacsclient does not
know about notmuch-test-progn. This leads to test suite failure due to
time out on any emacs test.