testing: make Eq(nan,<meh>) return False, Ne(nan, <meh>) return True

Not necessarily intended for a merge...
@asmeurer said (in $7774) "I always felt it led to more complications than were necessary. " What do we gain from (IMO) this seemingly obtuse result that nan == nan is false? Or wait -- is it Eq(nan, nan) that is false but nan == nan that is true? I just looked at another PR by you where you are trying to make things like Lt the same as <. How does this PR fit in with that idea?