Add functional tests

[LexManos]

Same caveats as the JarJar, couple hacks to make it work in environments that don't run gradle for every task.

Right now there are 3 tests, with bot kotlin and groovy variants.
makeFieldAccessible - Tests making a field accessible and trys compiling the code
makeMethodAccessible - Tests making a method accessible and trys compiling the code

And makeTransitiveAccessible, which uses the constraint example you wrote for at-grdle-demo, but I am not sure what you were trying to illustrate there. I thought it was showing how to transform a transitive dependency without polluting your pom/module file with that transitive.
But it doesn't work because it triggers reportDependencyConstraintWithoutVersion.

So that one is disabled for now, until i'm told what the purpose was.