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

compiler/testData/codegen/controlStructures/forNullableIntArray.kt

@@ -9,7 +9,6 @@ fun box() : String {
     for (el in b) {
         sum = sum + (el ?: 0)
     }
-    System.out?.println(sum)
     if(sum != 10) return "b failed"
 
     return "OK"