t/test-lib.sh: exit with small negagive int is ok with test_must_fail

t/test-lib.sh: exit with small negagive int is ok with test_must_fail

The test_must_fail function in test-lib.sh has been designed to
distinguish segmentation faults from controlled errors.  But in the
current implementation this only works if a git command does not return a
small negative value, like -1, -2 or -3.  But some git commands do.

Because any signal (like SIGSEGV) will result in an exit status
less than 193, this patch just adds a further check for the exit
status.

Signed-off-by: Stephan Beyer <s-beyer@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>