fix unused warnings in tests

Author: RalfJung

tests/compile-fail/deallocate-bad-alignment.rs

@@ -1,4 +1,4 @@
-use std::alloc::{alloc, dealloc, realloc, Layout};
+use std::alloc::{alloc, dealloc, Layout};
 
 // error-pattern: allocation has size 1 and alignment 1, but gave size 1 and alignment 2
 

tests/compile-fail/deallocate-bad-size.rs

@@ -1,4 +1,4 @@
-use std::alloc::{alloc, dealloc, realloc, Layout};
+use std::alloc::{alloc, dealloc, Layout};
 
 // error-pattern: allocation has size 1 and alignment 1, but gave size 2 and alignment 1
 

tests/compile-fail/deallocate-twice.rs

@@ -1,4 +1,4 @@
-use std::alloc::{alloc, dealloc, realloc, Layout};
+use std::alloc::{alloc, dealloc, Layout};
 
 // error-pattern: dereferenced after this allocation got freed
 

tests/compile-fail/reallocate-bad-size.rs

@@ -1,4 +1,4 @@
-use std::alloc::{alloc, dealloc, realloc, Layout};
+use std::alloc::{alloc, realloc, Layout};
 
 // error-pattern: allocation has size 1 and alignment 1, but gave size 2 and alignment 1
 

tests/compile-fail/reallocate-change-alloc.rs

@@ -1,4 +1,4 @@
-use std::alloc::{alloc, dealloc, realloc, Layout};
+use std::alloc::{alloc, realloc, Layout};
 
 fn main() {
     unsafe {