Validation of json data against game initialization test

[Cervator]

It would be nice if we could run a validation test of some sort (unit test?) against module content like blocks or prefabs to verify that they actually have valid references.

Came up in a big way when we did the big extraction of stuff from engine to core while piles of modules retained their "engine:xyz" entries in blocks and prefabs. The game catches all that on initialization (and logs warnings, then continues), so we can probably catch it in some sort of initialization test.

Inter-module dependencies might be tricky, especially if we begin supporting optional module dependencies, but module solo builds in Jenkins do pull in dependency modules and make them available for compilation, while the whole engine harness is available to run unit tests depending on TerasologyTestingEnvironment (which starts up a limited game instance)

I've done terrible things with a local Groovy script to write out results in Checkstyle report format so I can show it in Jenkins. I bet we could do something similar and write out unit test reports or so