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/indices.kt

@@ -0,0 +1,9 @@
+fun box(): String {
+    val a = Array<Int>(5, {it})
+    val x = a.indices.iterator()
+    while (x.hasNext()) {
+        val i = x.next()
+        if (a[i] != i) return "Fail $i ${a[i]}"
+    }
+    return "OK"
+}