introduced abstract TestCase for common fixture code reuse
working unit test using integration-test module source files

also clean up test/data directory in integration-plugin module
also introduced abstract classes for factorize unit test code
set up a working use of the source in integration-test module in a plugin unit test
wip creating a unit test for resolution of class with ElementFinder implementation

the current code is only an abstract class because we couldn't make it work
not sure the current approach will work either
add TODOs for unit testing Psi parsing and asking question on forum
add unit test for Psi tree parsing, validation and source generation
add unit test for Psi parsing, validation and source generation for mapper-enum

add an abstract class for reuse for other modules of integration-test
unit test is ok for @Mapper classes EnumInstancedGuavaFunction and GenericsEverywhere but not yet for ReturnArray