Update to conformance test suite

[anvacaru]

Adding support for the new execution-spec-tests conformance suite of the EVM.
Creating a new dir under tests/execution-spec-tests with:

1. a new get-execution-spec-tests.sh script inspired by the one provided in the documentation (source) that downloads a given version of the JSON fixtures from the official GH release page.
2. a new kevm-pyk/src/tests/integration/test_execution_spec_tests.py pytest file that executes four categories of tests: blockchain, blockchain-engine, state and transactions. Duplicated logic from the old kevm-pyk/src/tests/integration/test_conformance.py has been refactored under kevm-pyk/src/tests/utils.py
3. new execution-spec-tests/failing.llvm file.
4. I added the test-fixtures and fixtures-failing-list Makefile rules that download the JSON fixtures. They are used on CI.

[ehildenb]

So now the test-integration will run both the old tests in the old format, and the new ones? And I guess the plan is that eventually we'll pass all the new ones and phase out the old ones?

[ehildenb]

LGTM!