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/forLoopMemberExtensionNext.kt

@@ -12,12 +12,14 @@ class X {
 
     fun test() {
         for (i in C()) {
-            System.out.println(i)
+            foo(i)
         }
     }
 
 }
 
+fun foo(x: Int) {}
+
 fun main(args: Array<String>) {
     X().test()
-}
+}