Use "rustc" for testing Command args

"echo" is not an application on Windows so `Command` tests could fail even if that's not what's being tested for.

library/std/src/process/tests.rs

@@ -297,23 +297,23 @@ fn test_interior_nul_in_progname_is_error() {
 
 #[test]
 fn test_interior_nul_in_arg_is_error() {
-    match Command::new("echo").arg("has-some-\0\0s-inside").spawn() {
+    match Command::new("rustc").arg("has-some-\0\0s-inside").spawn() {
         Err(e) => assert_eq!(e.kind(), ErrorKind::InvalidInput),
         Ok(_) => panic!(),
     }
 }
 
 #[test]
 fn test_interior_nul_in_args_is_error() {
-    match Command::new("echo").args(&["has-some-\0\0s-inside"]).spawn() {
+    match Command::new("rustc").args(&["has-some-\0\0s-inside"]).spawn() {
         Err(e) => assert_eq!(e.kind(), ErrorKind::InvalidInput),
         Ok(_) => panic!(),
     }
 }
 
 #[test]
 fn test_interior_nul_in_current_dir_is_error() {
-    match Command::new("echo").current_dir("has-some-\0\0s-inside").spawn() {
+    match Command::new("rustc").current_dir("has-some-\0\0s-inside").spawn() {
         Err(e) => assert_eq!(e.kind(), ErrorKind::InvalidInput),
         Ok(_) => panic!(),
     }