FIX: consistently ignore self loops in chordal graphs

[MridulS]

In https://github.com/networkx/networkx/pull/6563 we decided to ignore self loops in chordal graphs but we weren't really testing for the implementation. The self_loop_G was both a chordal and not a chordal graph.

I've updated the test which checks that we ignore self loops which checking for chordality.

[rossbar]

@MridulS I think this one would benefit from your insight on what the desired behavior is re: the removed test!