Refactored tests using Array constructor:

Some moved to tests with stdlib
Some changed to use arrayOfNulls

compiler/testData/codegen/box/arrays/arrayGetAssignMultiIndex.kt

@@ -1,11 +0,0 @@
-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]
-}