tests/float/inf_nan_arith.py: Include -inf in argument combos.

This adds tests for, eg, -inf + inf which should be nan.

Signed-off-by: Damien George <damien@micropython.org>

tests/float/inf_nan_arith.py

@@ -3,7 +3,7 @@
 inf = float("inf")
 nan = float("nan")
 
-values = (-2, -1, 0, 1, 2, inf, nan)
+values = (-2, -1, 0, 1, 2, inf, -inf, nan)
 
 for x in values:
     for y in values: