Update test/pretty output for edition preludes.

src/test/pretty/asm.pp

@@ -2,7 +2,7 @@
 #![no_std]
 #![feature(asm)]
 #[prelude_import]
-use ::std::prelude::v1::*;
+use ::std::prelude::rust_2015::*;
 #[macro_use]
 extern crate std;
 

src/test/pretty/cast-lt.pp

@@ -1,7 +1,7 @@
 #![feature(prelude_import)]
 #![no_std]
 #[prelude_import]
-use ::std::prelude::v1::*;
+use ::std::prelude::rust_2015::*;
 #[macro_use]
 extern crate std;
 // pretty-compare-only

src/test/pretty/dollar-crate.pp

@@ -1,7 +1,7 @@
 #![feature(prelude_import)]
 #![no_std]
 #[prelude_import]
-use ::std::prelude::v1::*;
+use ::std::prelude::rust_2015::*;
 #[macro_use]
 extern crate std;
 // pretty-compare-only

src/test/pretty/expanded-and-path-remap-80832.pp

@@ -1,7 +1,7 @@
 #![feature(prelude_import)]
 #![no_std]
 #[prelude_import]
-use ::std::prelude::v1::*;
+use ::std::prelude::rust_2015::*;
 #[macro_use]
 extern crate std;
 // Test for issue 80832

src/test/pretty/issue-12590-c.pp

@@ -1,7 +1,7 @@
 #![feature(prelude_import)]
 #![no_std]
 #[prelude_import]
-use ::std::prelude::v1::*;
+use ::std::prelude::rust_2015::*;
 #[macro_use]
 extern crate std;
 // pretty-compare-only

src/test/pretty/issue-4264.pp

@@ -1,5 +1,5 @@
 #[prelude_import]
-use ::std::prelude::v1::*;
+use ::std::prelude::rust_2015::*;
 #[macro_use]
 extern crate std;
 // pretty-compare-only