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/patternMatching/nullableWhen.kt

@@ -1,3 +1,6 @@
+// KT-2148
+
+
 fun f(p: Int?): Int {
     return when(p) {
         null -> 3