change anon_pipe_spawn_echo to use printf(1)

The illumos version of `echo(1)` doesn't support `-n`, so it interprets
it literally as part of the string to echo, making the test fail. I've
changed it to use `printf(1)` with no flags args to get the same
behavior.

I also changed the name of the test, since it doesn't actually spawn
`echo(1)` anymore :)

tokio/tests/net_unix_pipe.rs

@@ -457,15 +457,14 @@ async fn anon_pipe_simple_send() -> io::Result<()> {
 }
 
 #[tokio::test]
-async fn anon_pipe_spawn_echo() -> std::io::Result<()> {
+async fn anon_pipe_spawn_printf() -> std::io::Result<()> {
     use tokio::process::Command;
 
     const DATA: &str = "this is some data to write to the pipe";
 
     let (tx, mut rx) = pipe::pipe()?;
 
-    let status = Command::new("echo")
-        .arg("-n")
+    let status = Command::new("printf")
         .arg(DATA)
         .stdout(tx.into_blocking_fd()?)
         .status();