gtestdbus: Fix watcher crash on FreeBSD

gtestdbus: Fix watcher crash on FreeBSD

In file gio/gtestdbus.c, function watch_parent, there is a loop which
waits for commands sent from the parent process and kills all processes
recorded in 'pids_to_kill' array on parent process exit. The detection
of parent process exit is done by calling g_poll and checking whether
the returned event is G_IO_HUP. However, 'revents' is a bit mask, and
we should use a bitwise-AND check instead of the equality check here.

It seems to work fine on Linux, but it fails on FreeBSD because the
g_poll returns both G_IO_IN and G_IO_HUP on pipe close. This means the
watcher process continues waiting for commands after the parent process
exit, and g_io_channel_read_line returns G_IO_STATUS_EOF with 'command'
set to NULL. Then the watcher process crashes with segfault when calling
sscanf because 'command' is NULL. Since the test result is already
reported by the parent process as 'OK', this kind of crash is likely to
be unnoticed unless someone checks dmesg messages after the test:

pid 57611 (defaultvalue), uid 1001: exited on signal 11
pid 57935 (actions), uid 1001: exited on signal 11
pid 57945 (gdbus-bz627724), uid 1001: exited on signal 11
pid 57952 (gdbus-connection), uid 1001: exited on signal 11
pid 57970 (gdbus-connection-lo), uid 1001: exited on signal 11
pid 57976 (gdbus-connection-sl), uid 1001: exited on signal 11
pid 58039 (gdbus-exit-on-close), uid 1001: exited on signal 11
pid 58043 (gdbus-exit-on-close), uid 1001: exited on signal 11
pid 58047 (gdbus-exit-on-close), uid 1001: exited on signal 11
pid 58051 (gdbus-exit-on-close), uid 1001: exited on signal 11
pid 58055 (gdbus-export), uid 1001: exited on signal 11
pid 58059 (gdbus-introspection), uid 1001: exited on signal 11
pid 58065 (gdbus-names), uid 1001: exited on signal 11
pid 58071 (gdbus-proxy), uid 1001: exited on signal 11
pid 58079 (gdbus-proxy-threads), uid 1001: exited on signal 11
pid 58083 (gdbus-proxy-well-kn), uid 1001: exited on signal 11
pid 58091 (gdbus-test-codegen), uid 1001: exited on signal 11
pid 58095 (gdbus-threading), uid 1001: exited on signal 11
pid 58104 (gmenumodel), uid 1001: exited on signal 11
pid 58108 (gnotification), uid 1001: exited on signal 11
pid 58112 (gdbus-test-codegen-), uid 1001: exited on signal 11
pid 58116 (gapplication), uid 1001: exited on signal 11
pid 58132 (dbus-appinfo), uid 1001: exited on signal 11

If the watcher process crashes before killing the dbus-daemon process
spawned by the parent process, the dbus-daemon process will keep running
after all tests complete. Due to the implementation of 'communicate'
function in Python subprocess, it causes meson to crash. 'communicate'
assumes the stdout and stderr pipes are closed when the child process
exits, but it is not true if processes forked by the child process
doesn't exit. It causes Python subprocess 'communicate' function to
block on the call to poll until the timeout expires even if the test
finishes in a few seconds. Meson assumes the timeout exception always
means the test is still running. It calls 'communicate' again and
crashes because pipes no longer exist.

https://gitlab.gnome.org/Infrastructure/GitLab/issues/286
https://github.com/mesonbuild/meson/issues/3967
https://bugs.python.org/issue30154