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/objects/importFromClassObject.kt

@@ -1,15 +0,0 @@
-import A.B.foo
-import A.B.x
-
-class A() {
-    class object {
-        class B() {
-            class object {
-                val x = 1
-                fun foo() = "2"
-            }
-        }
-    }
-}
-
-fun box() = if (x == 1 && foo() == "2") "OK" else "fail"