Run constant-time tests as part of test suite (except on rare platforms to be problematic)

[real-or-random]

See discussions here:

• https://gnusha.org/secp256k1/2024-07-01.log
• discussion: Config used by Core vs defaults and config in CI/testing  #1549

We want to exclude problematic platforms to address the concerns in #723 .

Rare platforms that were problematic in the past are

• s390,
• power9 (also http://gnusha.org/secp256k1/2020-01-11.log at 11:29 and 11:37 )

But all of these reports are old, in particular the second one is for the old ECDH code. It may be a good idea to start with an empty list and see if we'll actually get reports.