test: improve the test coverage

Author: Tellcts

src/data_structures/binary_search_tree.rs

@@ -250,6 +250,13 @@ mod test {
         );
         assert!(!tree.search(&"only a sith deals in absolutes"));
         assert!(!tree.search(&"you underestimate my power"));
+        assert!(!tree.search(&"apple pie"));
+
+        let empty_tree = BinarySearchTree::<i32>::new();
+        assert!(!empty_tree.search(&12));
+        assert!(!empty_tree.search(&-5));
+        assert!(!empty_tree.search(&0));
+        assert!(!empty_tree.search(&5));
     }
 
     #[test]
@@ -316,6 +323,10 @@ mod test {
             "your move"
         );
         assert!(tree.ceil(&"your new empire").is_none());
+
+        let empty_tree = BinarySearchTree::<i32>::new();
+        assert!(empty_tree.floor(&12).is_none());
+        assert!(empty_tree.ceil(&12).is_none());
     }
 
     #[test]