Elide existing main function when building a test runner. Issue #428

This prevents any defined main function from colliding with the one
synthesized for the test runner. This is not the best solution since it
doesn't compile a function the user defined, but I don't think it's likely to
be a problem in the near term.

src/comp/syntax/fold.rs

@@ -12,6 +12,7 @@ export dummy_out;
 export noop_fold_crate;
 export noop_fold_item;
 export noop_fold_expr;
+export noop_fold_mod;
 
 type ast_fold = @mutable a_f;