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

The only difference between the files is the presence/absence of

    #![feature(unix_sigpipe)]

attribute. Avoid duplication by using revisions instead.

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;