Allow defining E2E test in Jsonnet

[eyal-mor]

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 (or even BDD) style definitions for tests.

Motivation

Describe alternatives you've considered

Additional context