t/t3437: remove the dependency of 'expected-message' file from tests

t/t3437: remove the dependency of 'expected-message' file from tests

As it is currently implemented, it's too difficult to follow along and
remember the value of "expected-message" from test to test. It also
makes it difficult to extend tests or add new tests in between existing
tests without negatively impacting other tests.

Let's set up "expected-message" to the precise content needed by the
test, so that both the problems go away and also makes easier to run
tests selectively with '--run' or 'GIT_SKIP_TESTS'

Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Charvi Mendiratta <charvi077@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>