Refactored tests using Array constructor:

Some moved to tests with stdlib
Some changed to use arrayOfNulls

compiler/testData/codegen/boxWithStdlib/arrays/arrayGetMultiIndex.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] = "OK"
+    return s[-2, 2]
+}
+