[tor-dev] prob_distr.c: LogLogistic fails stochastic tests on 32-bits mingw

[George Kadianakis]

Hello Riastradh,

as discussed on IRC, Appveyor recently started failing the stochastic
tests of LogLogistic on 32-bit builds:
     https://github.com/torproject/tor/pull/576
     https://ci.appveyor.com/project/torproject/tor/builds/20897462

I managed to reproduce the breakage by cross-compiling Tor and running
the tests with wine, using this script of ahf: https://github.com/ahf/tor-win32/

Here are my findings:

The following two test cases are breaking 100% reproducibly:

    ok = test_stochastic_log_logistic_impl(M_E, 1e-1);
    ok = test_stochastic_log_logistic_impl(exp(-10), 1e-2);

The breakage seems to be because of the beta parameter. In particular,
it seems like the test will break with any beta <= 0.26, and will
succeed with a beta >= 0.27. The space in between is still unclear ;)

I haven't managed to figure out what's actually offending the test but I
can reproduce this so I can do some digging if you have any ideas.

FWIW, I haven't noticed any other stochastic test breakage.

Cheers!