t/t3800: do not use a temporary file to hold expected result.

t/t3800: do not use a temporary file to hold expected result.

It is a good practice to write program output to a temporary file
during the test, as it would allow easier postmortem when the tested
program does break.  But there is no benefit in writing the expected
output out to the temporary.

This actually fixes a bug in check_verify_failure() routine.
The intention of the test seems to make sure the "git mktag" command
fails, and it spits out the expected error message.  But if the
command did not fail as expected, the shell function as originally
written would not have detected the failure.

Signed-off-by: Junio C Hamano <gitster@pobox.com>