Mark cfg test xfail-fast since check-fast doesn't understand compile-flags.

Author: luqmana

src/test/run-pass/cfgs-on-items.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// xfail-fast
 // compile-flags: --cfg fooA --cfg fooB
 
 // fooA AND !bar