Run more iterations of run_field_misc
Run more iterations of run_field_misc

At count=64, this makes the test take around 1% of the total time.

bitcoin/secp256k1,bitcoin/secp256k1,ElementsProject/secp256k1-zkp,bitcoin-core/secp256k1,bitcoin/secp256k1,bitcoin/secp256k1,bitcoin-core/secp256k1,bitcoin/secp256k1,peterdettman/secp256k1,peterdettman/secp256k1,ElementsProject/secp256k1-zkp