robot-name: remove redundant tests (#1983)

[no important files changed]

part of #1824

This is another case where I think it makes sense to actually keep the
util function around. It produces a good error message and in
combination with the body of the test function, users should have enough
information.

As the comment in the diff already states, the removed tests are
redundant because the type system prevents such errors anyway.

exercises/practice/robot-name/tests/robot-name.rs

@@ -12,27 +12,12 @@ fn assert_name_matches_pattern(n: &str) {
     );
 }
 
-fn assert_name_is_persistent(r: &robot::Robot) {
-    // The type system already proves this, but why not.
-    let n1 = r.name();
-    let n2 = r.name();
-    let n3 = r.name();
-    assert_eq!(n1, n2);
-    assert_eq!(n2, n3);
-}
-
 #[test]
 fn name_should_match_expected_pattern() {
     let r = robot::Robot::new();
     assert_name_matches_pattern(r.name());
 }
 
-#[test]
-#[ignore]
-fn name_is_persistent() {
-    assert_name_is_persistent(&robot::Robot::new());
-}
-
 #[test]
 #[ignore]
 fn different_robots_have_different_names() {
@@ -63,14 +48,6 @@ fn new_name_should_match_expected_pattern() {
     assert_name_matches_pattern(r.name());
 }
 
-#[test]
-#[ignore]
-fn new_name_is_persistent() {
-    let mut r = robot::Robot::new();
-    r.reset_name();
-    assert_name_is_persistent(&r);
-}
-
 #[test]
 #[ignore]
 fn new_name_is_different_from_old_name() {