gtestutils: deprecate g_test_trap_fork()

https://bugzilla.gnome.org/show_bug.cgi?id=679683

glib/tests/testing.c

@@ -85,6 +85,9 @@ test_timer (void)
   g_test_maximized_result (5, "bogus-quantity: %ddummies", 5); /* simple API test */
 }
 
+#ifdef G_OS_UNIX
+G_GNUC_BEGIN_IGNORE_DEPRECATIONS
+
 /* fork out for a failing test */
 static void
 test_fork_fail (void)
@@ -127,6 +130,9 @@ test_fork_timeout (void)
   g_assert (g_test_trap_reached_timeout());
 }
 
+G_GNUC_END_IGNORE_DEPRECATIONS
+#endif /* G_OS_UNIX */
+
 static void
 test_subprocess_fail_child (void)
 {