Segfault in fq_default tests on Cygwin

[fredrik-johansson]

Following #2047 and/or #2050 there is a problem in the fq_default tests on Cygwin/GCC: https://github.com/flintlib/flint/actions/runs/10773322840/job/29872862221

Not sure why; the new code doesn't look platform-dependent in any way.

[albinahlback]

Could be that it invokes some bug in some dependency of fq_default. Hard to know though, especially since Cygwin is almost never used.

[albinahlback]

I think there might be a bug in fq_default_ctx_init_randtest, but I have to investigate this more to give a definite answer.

EDIT: It is the test for fq_default_poly_evaluate that crashes. See CI runner.