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/arrays/iteratorByteArrayNextByte.kt

@@ -0,0 +1,10 @@
+fun box(): String {
+    val a = ByteArray(5)
+    val x = a.iterator()
+    var i = 0
+    while (x.hasNext()) {
+        if (a[i] != x.nextByte()) return "Fail $i"
+        i++
+    }
+    return "OK"
+}