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;
 

tests/ui/attributes/unix_sigpipe/unix_sigpipe-only-feature.rs

@@ -1,13 +0,0 @@
-//@ run-pass
-//@ aux-build:sigpipe-utils.rs
-
-#![feature(unix_sigpipe)]
-
-fn main() {
-    extern crate sigpipe_utils;
-
-    // Only #![feature(unix_sigpipe)] is enabled, not #[unix_sigpipe = "..."].
-    // This shall not change any behavior, so we still expect SIGPIPE to be
-    // ignored
-    sigpipe_utils::assert_sigpipe_handler(sigpipe_utils::SignalHandler::Ignore);
-}