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/arrayGetAssignMultiIndex.kt

@@ -0,0 +1,11 @@
+fun Array<String>.get(index1: Int, index2: Int) = this[index1 + index2]
+fun Array<String>.set(index1: Int, index2: Int, elem: String) {
+    this[index1 + index2] = elem
+}
+
+fun box(): String {
+    val s = Array<String>(1, { "" })
+    s[1, -1] = "O"
+    s[2, -2] += "K"
+    return s[-3, 3]
+}