tests: Fix flaky socket-service test caused by GTask scheduling
tests: Fix flaky socket-service test caused by GTask scheduling

On about 1 in 3 test runs, the socket-service would fail with the
ref_count assertion in connection_cb() failing (the ref_count would be 3
rather than the expected 2).

This was happening because the GTask from
g_socket_listener_accept_socket_async() now always takes at least one
main context iteration to return a result (whereas before
6f3d57d2ee271017f37ebdceb1e15efa7ce0ac49 it might have taken zero), but
the ref_count can drop below 3 before the process of returning a result
starts. During the process of returning a result, the ref_count
temporarily increases again, which is what was breaking the test.

Fix this by waiting for one more main context iteration. This is a bit
of a hack, but the real fix would be to expose the outstanding_accept
boolean from GSocketService as public API (which the test can
interrogate), and that seems too much like exposing internal state.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Helps: #1679

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib