termwiz: fixup key_tester example on macOS

We need to set the terminal to blocking mode when we want poll_input
to block forever, otherwise we immediately receive WouldBlock error
response and quit the program.

termwiz/src/terminal/unix.rs

@@ -449,6 +449,12 @@ impl Terminal for UnixTerminal {
             },
         ];
 
+        self.read.set_blocking(if wait.is_none() {
+            Blocking::Wait
+        } else {
+            Blocking::DoNotWait
+        })?;
+
         let poll_result = unsafe {
             poll(
                 pfd.as_mut_ptr(),