Simplify test. No functional change intended.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207623 91177308-0d34-0410-b5e6-96231b3b80d8

test/Frontend/windows-nul.c

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -dM -E nul -o /dev/null
+// RUN: %clang_cc1 nul
 
 // REQUIRES: system-windows