make tests compatible with 2018 edition

tests/compile-fail/ctlz_nonzero.rs

@@ -8,7 +8,7 @@ mod rusti {
 
 pub fn main() {
     unsafe {
-        use rusti::*;
+        use crate::rusti::*;
 
         ctlz_nonzero(0u8); //~ ERROR constant evaluation error: ctlz_nonzero called on 0
     }

tests/compile-fail/cttz_nonzero.rs

@@ -8,7 +8,7 @@ mod rusti {
 
 pub fn main() {
     unsafe {
-        use rusti::*;
+        use crate::rusti::*;
 
         cttz_nonzero(0u8); //~ ERROR constant evaluation error: cttz_nonzero called on 0
     }

tests/run-pass-fullmir/foreign-fn-linkname.rs

@@ -9,14 +9,16 @@
 // except according to those terms.
 
 //ignore-windows: Uses POSIX APIs
-#![feature(libc)]
+
+#![feature(libc, extern_crate_item_prelude)]
+#![allow(unused_extern_crates)] // rustc bug https://github.com/rust-lang/rust/issues/56098
 
 extern crate libc;
+
 use std::ffi::CString;
 
 mod mlibc {
     use libc::{c_char, size_t};
-
     extern {
         #[link_name = "strlen"]
         pub fn my_strlen(str: *const c_char) -> size_t;

tests/run-pass-fullmir/memchr.rs

@@ -1,6 +1,5 @@
 #![feature(slice_internals)]
 
-extern crate core;
 use core::slice::memchr::{memchr, memrchr};
 
 // test fallback implementations on all platforms

tests/run-pass/intrinsics-integer.rs

@@ -23,7 +23,7 @@ mod rusti {
 
 pub fn main() {
     unsafe {
-        use rusti::*;
+        use crate::rusti::*;
 
         assert_eq!(ctpop(0u8), 0); assert_eq!(ctpop(0i8), 0);
         assert_eq!(ctpop(0u16), 0); assert_eq!(ctpop(0i16), 0);