bst_valid_challenge's one test case is wrong

[goalong]

hi, I noticed that test case below is not a valid binary search tree, it might be valid if we delete the node 6, but in your challenge notebook marked it as valid. I'm not sure I should delete the node 6 and send a pr or just submit an issue, so I just submit an issue.
Valid:

      5
    /   \
   5     8
  / \   /
 4   6 7

[donnemartin]

Hi @goalong, nice catch! Yes I think we should delete node 6 in the test cases and unit tests.

Are you interested in a PR?

[goalong]

Sure, I will send a PR soon.

[girishkumbhar]

Ok, Send PR