use SIGKILL in remap_dead_pid test

For some undiagnosed reason, this test hangs for me when using SIGINT. Since we
always want to kill this process anyway, use SIGKILL instead.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>