test: Don't test that tags with zero variants are supported

Author: pcwalton

src/test/compile-fail/tag-type-args.rs

@@ -1,6 +1,6 @@
 // error-pattern: Wrong number of type arguments
 
-tag quux<T> { }
+tag quux<T> { bar }
 
 fn foo(c: quux) { assert (false); }