Tests: nan not orderable

In PR #7776, @Tempel thought `nan` should not be orderable, by analogy to `Lt(I,1)`.

Here's a test for that.  Currently it passes.  The test only includes the long forms like `Lt` not the symbolic shortcuts `<`.

The shortcuts actually fail, but that's PR #7783.
Probably, you can use shorter aliases (Ge, Lt and so on).