Rename *.jet -> *.kt in codegen tests
Refactor codegen tests

- initialize environment only once in setUp()
- add comments on why some tests are disabled
- modify and rename some tests
- re-enable now working tests
- extract some tests into files with box()
- remove useless 'throws' declarations and commented code
Remove tests on RTTI

It was never implemented
Refactor codegen tests: no special box() result

box() shall always return "OK"
Remove obsolete special files from android tests