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/longAsIndex.kt

@@ -0,0 +1,9 @@
+fun IntArray.set(index: Long, elem: Int) { this[index.toInt()] = elem }
+fun IntArray.get(index: Long) = this[index.toInt()]
+
+fun box(): String {
+    var l = IntArray(1)
+    l[0.toLong()] = 4
+    l[0.toLong()] += 6
+    return if (l[0.toLong()] == 10) "OK" else "Fail"
+}