Removed manual checks to just using clap instead

Also added tests for this.

Signed-off-by: Hanif Bin Ariffin <hanif.ariffin.4326@gmail.com>

src/uu/tr/src/tr.rs

@@ -364,6 +364,7 @@ pub fn uu_app() -> App<'static, 'static> {
             Arg::with_name(options::SETS)
                 .multiple(true)
                 .takes_value(true)
+                .min_values(1)
                 .max_values(2),
         )
 }