Unused import in tests

[opilar]

λ ~/rust/rustfmt/ master rustup run nightly cargo test
   Compiling rustfmt-nightly v0.2.7 (file:///Users/opilar/rust/rustfmt)
warning: unused import: `Write`
  --> tests/system.rs:19:47
   |
19 | use std::io::{self, BufRead, BufReader, Read, Write};
   |                                               ^^^^^
   |
   = note: #[warn(unused_imports)] on by default

warning: unused imports: `Command`, `Stdio`
  --> tests/system.rs:21:20
   |
21 | use std::process::{Command, Stdio};
   |                    ^^^^^^^  ^^^^^