unix_sigpipe: Combine unix_sigpipe-not-used.rs and unix_sigpipe-only-feature.rs

The only difference between the files is the present of the feature
attribute. Avoid duplication by using revisions instead. Later commits
will make more use of revisions.

Author: Enselic

tests/ui/attributes/unix_sigpipe/unix_sigpipe-not-used.rs

@@ -1,6 +1,9 @@
+// revisions: with_feature without_feature
 // run-pass
 // aux-build:sigpipe-utils.rs
 
+#![cfg_attr(with_feature, feature(unix_sigpipe))]
+
 fn main() {
     extern crate sigpipe_utils;