testsuite: Re-xfail record-trailing-comma

It was xfailed before the other commits in this pull request,
so no big deal. I'll look into it later.

Author: catamorphism

src/test/pretty/record-trailing-comma.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// xfail-test
 // pp-exact
 struct Thing {
     x: int,