t: prepare `test_match_signal ()` calls for `set -e`

t: prepare `test_match_signal ()` calls for `set -e`

We have a couple of calls to `test_match_signal ()` where we execute a
Git command and expect it to die with a specific signal. These calls
will essentially execute the process in a subshell via `foo; echo $?`,
but as we expect `foo` to fail this will cause the overall subshell to
fail once we `set -e`.

Fix this issue by using `foo && echo 0 || echo $?` instead.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>