t7107: don't use test_must_fail()

t7107: don't use test_must_fail()

We had a `test_must_fail verify_expect`. However, the git command in
verify_expect() was not expected to fail; the test_cmp() was the failing
command. Be more precise about testing failure by accepting an optional
first argument of '!' which causes the result of the file comparison to
be negated.

Signed-off-by: Denton Liu <redacted>
Signed-off-by: Junio C Hamano <redacted>