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/classes/nonnullarray.kt

@@ -1,8 +0,0 @@
-class A() {
-    class B(val i: Int) {
-    }
-
-    fun test() = Array<B> (10, { B(it) })
-}
-
-fun box() = if(A().test()[5].i == 5) "OK" else "fail"