app: Don't use the gimpdir for WM tests

The WM tests mess up the GIMP dir, in particular any sessionrc if it
exists, due to the interface never being shown. Make the WM test use a
tmp dir to avoid that.

app/tests/test-window-management.c

@@ -47,6 +47,9 @@ int main(int argc, char **argv)
 {
   int test_result;
 
+  /* Disable the user dir for this test */
+  g_setenv ("GIMP2_DIRECTORY", "/tmp/gimpdir", TRUE);
+
   g_type_init ();
   gtk_init (&argc, &argv);
   g_test_init (&argc, &argv, NULL);