tests: Add ui/attributes/unix_sigpipe/unix_sigpipe-str-list.rs

Which is a variant of [`unix_sigpipe-list.rs`][1] but where a string is
used instead of an identifier. This makes it more similar to the proper
form `#[unix_sigpipe = "sig_dfl"]` and thus more likely to be written by
users by mistake. Also rename the first test to be more in line with the
terminology of [The Reference][2].

[1]: https://github.com/rust-lang/rust/blob/master/tests/ui/attributes/unix_sigpipe/unix_sigpipe-list.rs
[2]: https://doc.rust-lang.org/reference/attributes.html#meta-item-attribute-syntax

tests/ui/attributes/unix_sigpipe/unix_sigpipe-ident-list.stderr

@@ -1,5 +1,5 @@
 error: malformed `unix_sigpipe` attribute input
-  --> $DIR/unix_sigpipe-list.rs:3:1
+  --> $DIR/unix_sigpipe-ident-list.rs:3:1
    |
 LL | #[unix_sigpipe(sig_dfl)]
    | ^^^^^^^^^^^^^^^^^^^^^^^^ help: must be of the form: `#[unix_sigpipe = "inherit|sig_ign|sig_dfl"]`

tests/ui/attributes/unix_sigpipe/unix_sigpipe-str-list.rs

@@ -0,0 +1,4 @@
+#![feature(unix_sigpipe)]
+
+#[unix_sigpipe("sig_dfl")] //~ error: malformed `unix_sigpipe` attribute input
+fn main() {}

tests/ui/attributes/unix_sigpipe/unix_sigpipe-str-list.stderr

@@ -0,0 +1,8 @@
+error: malformed `unix_sigpipe` attribute input
+  --> $DIR/unix_sigpipe-str-list.rs:3:1
+   |
+LL | #[unix_sigpipe("sig_dfl")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: must be of the form: `#[unix_sigpipe = "inherit|sig_ign|sig_dfl"]`
+
+error: aborting due to 1 previous error
+