Fix make check with builddir != srcdir

This broke when the tests were converted to be installable.
My apologies.

gio/tests/gdbus-connection-slow.c

@@ -224,7 +224,7 @@ main (int   argc,
   if (g_getenv ("G_TEST_DATA"))
     binpath = g_getenv ("G_TEST_DATA");
   else
-    binpath = ".";
+    binpath = BUILDDIR;
 
   g_test_init (&argc, &argv, NULL);