t/lib-git-daemon: fix signal checking

Test scripts checking 'git daemon' stop the daemon with a TERM signal, and the 'stop_git_daemon' helper checks the daemon's exit status to make sure that it indeed died because of that signal. This check is bogus since 03c39b3458 (t/lib-git-daemon: use test_match_signal, 2016-06-24), for two reasons: - Right after killing 'git daemon', 'stop_git_daemon' saves its exit status in a variable, but since 03c39b3458 the condition checking the exit status looks at '$?', which at this point is not the exit status of 'git daemon', but that of the variable assignment, i.e. it's always 0. - The unexpected exit status should abort the whole test script with 'error', but it doesn't, because 03c39b3458 forgot to negate 'test_match_signal's exit status in the condition. This patch fixes both issues. Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>