tests: Fix a potential race condition in pollable test
This may have been causing an intermittent failure of the pollable test on BSD, where updating the readable status of a socket takes a bit longer than on Linux.
GLib-GIO-DEBUG: 16:06:41.235: GSocketClient: Starting application layer connection
GLib-GIO-DEBUG: 16:06:41.235: GSocketClient: Connection successful!
Bail out! GLib-GIO:ERROR:../gio/tests/pollable.c:73:check_source_readability_callback: assertion failed (readable == expected): (0 == 1)
I have not debugged the test on BSD, though, so this is only a guess.
See https://gitlab.gnome.org/GNOME/glib/-/jobs/2022087
Signed-off-by: Philip Withnall pwithnall@endlessos.org