Remove code duplication from test_blockchain_json

[jangko]

current situation:

test_blockchain_json.nim have it's own code for block validation, and nimbus has it's own code for block validation. The reason is, in the past nimbus block validation is a mess, so test_blockchain_json was developed in parallel as a debugging tool too and some of the code is ported back to nimbus or code from nimbus copied to test_blockchain_json.

why it is bad:

• if you change/fix code in one place and forget to change/fix the other place, weird error will greet you.
• test_block_chain is meant to be a test tool, but it is very complicated and bloated.
• as shown by graphql test suite, the same test data from EF test suite can use nimbus block validation. The resulting test code is much leaner and simpler to understand.