🔀(gentest): use jinja2 templates to structure python output

[danceratopz]

Jinja2 is a very powerful templating library that we started using in the doc flow for the generated "Test Case Reference" output pages, see #801 and ./docs/templates.

I think that .py.j2 templates could be an excellent way to write the output from gentest. This will be particularly helpful, when/if we create blockchain_tests with an arbitrary number of blocks, each containing an arbitrary number of transactions. We could have a template for a state_test and one for blockchain_test. Jinja2 also allows base templates.

[raxhvl]

Hey @danceratopz I'm starting with this.