System should not be an allowed enum name

Using an enum name of `system` can cause problems with `Kernel#system` which is dangerous and things could get weird. This adds `system` to the blacklisted class methods. The `detect_enum_conflict` method wasn't checking `enum_name` just the `method_name`.

I also updated the tests to include `system`.

Fixes #16288
FWIW this is also used to prevent named scopes. https://github.com/rails/rails/blob/7552dc1caba76d3dbbe6714e1aba5b93a22019b0/activerecord/lib/active_record/scoping/named.rb#L142-L143