This is a variant toggle, not an extra feature, so it should be tested
separately.
We test most of the effect of MBEDTLS_ENTROPY_FORCE_SHA256 (namely,
using SHA-256 in the entropy module) when we test the library with the
SHA512 module disabled (which we do at least via depends-hashes.pl).
This commit removes testing of the MBEDTLS_ENTROPY_FORCE_SHA256 option
itself, which should be added separately.