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/functions/defaultargs3.kt

@@ -4,7 +4,6 @@ class C() {
 
     fun testReceiver() : String {
         val res : String = "mama".toMyPrefixedString("111", "222")
-        System.out?.println(res)
         return res
     }
 

compiler/testData/codegen/functions/nothisnoclosure.kt

@@ -1,7 +1,9 @@
+fun foo(x: Int) {}
+
 fun loop(var times : Int) {
    while(times > 0) {
         val u : (value : Int) -> Unit = {
-            System.out?.println(it)
+            foo(it)
         }
         u(times--)
    }