Patch CBor round-trip QCheck test to hold for nan's too

[jmid]

The forthcoming QCheck 0.26 release ocaml/opam-repository#28148 changes the distribution of the float generator to avoid blind spots: c-cube/qcheck#350

As a consequence, the existing round-trip property-based test for CBor now fails to hold, as nan = nan doesn't hold, resulting in, e.g.:

File "tests/core/dune", line 2, characters 7-8:
2 |  (name t)
           ^
seed: 9590b92181c82cc2
testing containers: running 1219 tests…
FAILED: (test :file 'tests/core/t_cbor.pp.ml' :n 0)
1219 tests done in 4.880s
ERROR (1 failures)

========
failed (test :file 'tests/core/t_cbor.pp.ml' :n 0):
failed on instances:
  `{-nan: ""}` (after 25 shrink steps)
roundtrip failed: from {-nan: ""} to {-nan: ""}

This PR patches the round-trip property with a recursively defined equality predicate, falling back on Float.equal to compare nans (which has the desired behaviour).

[c-cube]

excellent catch! 😍 Thank you!

[c-cube]

thank you :)