Adjust test_FI_div_zero_result_top QCheck test property to account for the 0/0 case

[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, an existing goblint analyzer test now fails to hold, resulting in:

File "src/core/QCheck2.ml", line 2005, characters 1-1:
Error: :1:floatDomainTest:1:float_interval_qcheck32:2:test_FI_div_zero_result_top (in the code).


test `test_FI_div_zero_result_top` failed on ≥ 1 cases: <instance>

This PR first patches the arbitrary generator to use the existing show function to print counterexamples:

File "src/core/QCheck2.ml", line 2005, characters 1-1:
Error: :1:floatDomainTest:1:float_interval_qcheck32:2:test_FI_div_zero_result_top (in the code).


test `test_FI_div_zero_result_top` failed on ≥ 1 cases: [0.,0.]

Second, it patches the property of the mentioned QCheck test to account for the [0.;0.] / [0.;0.] case.
Here, I was guided by the unit test above already exercising this corner case:

analyzer/tests/unit/cdomains/floatDomainTest.ml

Lines 103 to 109 in f2c4520

	let test_FI_div_specific _ =
	let (/) = FI.div in
	let (=) a b = assert_equal b a in
	begin
	fi_zero / fi_one = fi_zero;
	(FI.of_const 2.) / fi_zero = FI.top ();
	fi_zero / fi_zero = FI.nan ();

In retrospect, I can see the arbitrary () generator produces random intervals by mapping convert_arb over a pair of random floats. The chance of producing a unit (const) interval with that strategy is close to zero. 🤔
I believe the change of distribution (and the ability of emitting nan) is the reason why the failure started to trigger with QCheck 0.26:

# min nan 0.;;
- : float = 0.
# max nan 0.;;
- : float = 0.

I therefore encourage you to investigate the distribution of the generator. I just adjusted the output of collect stats in QCheck 0.26 c-cube/qcheck#351 which lends itself to a nice "top", "bot", "const", "non-const" classification... 🤓 😅

[sim642]

Thanks a lot for looking so deep into our tests!