It might fail, for example with "Error, -bsaves makes sense in batch mode only"
error, and then all following tests also exhibit that error - which is not
detected, because those tests are expected to return an error code.

If the random sigma value fails, then try the next one (sigma+1),
and so on, instead of drawing a new random value. For small n,
this will avoid failures (for n=5, the probability of failure
with param=1 goes down from 1/150 to 0).

("make longcheck" with --enable-valgrind-tests)

(see #21867)

(Brian Gladman only maintains build.vs)

https://sympa.inria.fr/sympa/arc/ecm-discuss/2024-06/msg00001.html

Any difference term index > 14 will corrupt the chain state array.

Nested parallel regions incur large overhead by issuing a huge number of futex syscalls.
Fixes #21873