Silence Clippy warning

In newer version of Rust, Clippy complains about us potentially leaking
a process in our test code. The code is working exactly as intended, so
silence the warning.

src/tty/linux.rs

@@ -157,6 +157,7 @@ mod tests {
       return;
     }
 
+    #[allow(clippy::zombie_processes)]
     fn test(stdin: process::Stdio, redirection: &str) {
       let mut child = process::Command::new("sh")
         .stdin(stdin)