Allow defining E2E test in Jsonnet

## Summary

Although YAML is a great description language, when a large enough test set is required, things get out of hand.

The CLI tool already support Jsonnet, so it shouldn't be very hard to both support it as a renderer and provide a simple library to define [TDD](https://en.wikipedia.org/wiki/Test-driven_development) (or even [BDD](https://en.wikipedia.org/wiki/Behavior-driven_development)) style definitions for tests.

## Motivation

## Describe alternatives you've considered

## Additional context

Start by tracing the CLI's existing Jsonnet support and comparing it with how YAML test definitions are handled. The issue does not name files, tests, or an agreed design; done would require a defined Jsonnet renderer and a clear test-definition library scope.

Written by the indexing model from the issue text.