✨(gentest): use `evm_bytes` to generate python opcode in tests

Currently, evm code is written as a hex string in the generated test. This could be made more readable by converting the bytecode to the `Opcodes` mini-lang using `evm_bytes` (as used here):
https://github.com/ethereum/execution-spec-tests/blob/802467c890e55cac89ad903585f01e440f701728/tests/shanghai/eip3855_push0/test_push0.py#L61

The first todo would be probably to check whether `gentest` correctly generates tests that have a smart contract interaction :slightly_smiling_face:

I think it should be enabled by default, but can be disabled via an option.

Note: `evm_bytes_to_python` was renamed to `evm_bytes` in ethereum/execution-spec-tests#844