Allow slightly too small poll duration in /socket/timed_wait test

Sometimes the poll duration in the /socket/timed_wait test is slightly lower
than the requested 100000, causing failures like

ERROR:/build/buildd/glib2.0-2.33.2/./gio/tests/socket.c:619:test_timed_wait:
  assertion failed (poll_duration > = 100000): (99240 >= 100000)
FAIL

Adjust the test to also allow some jitter in the "too small" direction, similar
to the already existing span for "slightly too large".

https://bugzilla.gnome.org/show_bug.cgi?id=678881

gio/tests/socket.c

@@ -616,7 +616,7 @@ test_timed_wait (void)
   g_clear_error (&error);
   poll_duration = g_get_monotonic_time () - start_time;
 
-  g_assert_cmpint (poll_duration, >=, 100000);
+  g_assert_cmpint (poll_duration, >=, 98000);
   g_assert_cmpint (poll_duration, <, 110000);
 
   g_socket_close (client, &error);