Fix must_use lint for command exec tests

Author: c-ryan747

tests/ui/command/command-exec.rs

@@ -26,23 +26,23 @@ fn main() {
 }
 
 "exec-test2" => {
-Command::new("/path/to/nowhere").exec();
+let _ = Command::new("/path/to/nowhere").exec();
 println!("passed");
 }
 
 "exec-test3" => {
-Command::new(&me).arg("bad\0").exec();
+let _ = Command::new(&me).arg("bad\0").exec();
 println!("passed");
 }
 
 "exec-test4" => {
-Command::new(&me).current_dir("/path/to/nowhere").exec();
+let _ = Command::new(&me).current_dir("/path/to/nowhere").exec();
 println!("passed");
 }
 
 "exec-test5" => {
  env::set_var("VARIABLE","ABC");
-Command::new("definitely-not-a-real-binary").env("VARIABLE","XYZ").exec();
+let _ = Command::new("definitely-not-a-real-binary").env("VARIABLE","XYZ").exec();
 assert_eq!(env::var("VARIABLE").unwrap(),"ABC");
 println!("passed");
 }