Skip some test suites in constant-flow testing with Valgrind

[gilles-peskine-arm]

Running tests under a constant flow checker is only useful if the tests have annotations to declare some data as secret. Most test suites have no such annotations, so running them under a constant flow checker is a waste of time.

This is an issue with Valgrind because it's very slow. In particular, test_valgrind_constant_flow_psa tends to time out when the CI is being slow.

The goal of this issue is, in test_valgrind_constant_flow and test_valgrind_constant_flow_psa, to only run the test suites that are useful. Currently this is the ones that contain TEST_CF_SECRET (there are no auxiliary functions in tests/src that declare things as secret).

An optional bonus would be to reduce the duplication between test_valgrind_constant_flow and test_valgrind_constant_flow_psa. They only differ in whether MBEDTLS_USE_PSA_CRYPTO is enabled, and that only influences ssl_cf_hmac in (currently) test_suite_ssl. So as a bonus, one of the components should only run that one function.

We also do constant-flow testing with MSan, but that's considerably faster so I don't think we need such hacks there.