Add Tests tab to function nodes

[gregorius]

@AllanOricil

• jsonata could be used to bind env variables, but I would prefer writting tests with js if I ever need to inject env context in my tests. [OPTIONAL]

I like the idea that only "input - output" pairs are defined, no logic, no complexity. Allowing JSONata - even though it provides flexibility, the extra complexity (understanding tests if one isn't so firm on JSONata produces complexity) is a non-niceness!

Strangely I already suggested something similar --> Visual Unit Tests