Explain the unit test in ptrace.rs

Author: marmistrz

src/sys/ptrace.rs

@@ -253,6 +253,9 @@ mod tests {
 
     #[test]
     fn test_types() {
+        // c_long is implementation defined, so make sure
+        // no precision is lost
+        assert!(Word::min_value() <= usize::min_value());
         assert!(usize::max_value() <= Word::max_value());
     }
 }