t/lib-git-daemon: fix signal checking

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>