Simplify unit tests

[colmsnowplow]

In v1, we added a lot of unit test coverage, which is great. In some areas, however, they're overcomplicated. The transform/engine tests, and the trasformconfig validation tests, for example.

The latter of these has a tendency to panic because it relies on retrieving indexes from slices, when some changes mean they don't exist.

[TiganeteaRobert]

I agree that the tests are overcomplicated, I feel that besides simplifying the tests we should also better document them and refactor them so they are more readable. Going over the JS/Lua tests for example with no prior interaction with the custom transformations was a tad overwhelming so I would love to make them friendlier.