tests: Avoid clippy warning of unhandled I/O bytes

Fixes beta clippy issue:

error: read amount is not handled
    --> tests/integration.rs:2121:15
     |
2121 |         match pty.read(&mut buf) {
     |               ^^^^^^^^^^^^^^^^^^
     |
     = help: use `Read::read_exact` instead, or handle partial reads
note: the result is consumed here, but the amount of I/O bytes remains unhandled
    --> tests/integration.rs:2122:13
     |
2122 | /             Ok(_) => {
2123 | |                 let output = std::str::from_utf8(&buf).unwrap().to_string();
2124 | |                 match tx.send(output) {
2125 | |                     Ok(_) => (),
2126 | |                     Err(_) => break,
2127 | |                 }
2128 | |             }
     | |_____________^
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unused_io_amount
     = note: `#[deny(clippy::unused_io_amount)]` on by default

Signed-off-by: Bo Chen <chen.bo@intel.com>

tests/integration.rs

@@ -2119,7 +2119,7 @@ fn pty_read(mut pty: std::fs::File) -> Receiver<String> {
         thread::sleep(std::time::Duration::new(1, 0));
         let mut buf = [0; 512];
         match pty.read(&mut buf) {
-            Ok(_) => {
+            Ok(_bytes) => {
                 let output = std::str::from_utf8(&buf).unwrap().to_string();
                 match tx.send(output) {
                     Ok(_) => (),