Fix tests: "infix modifier required" and "operator modifier required" errors

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

@@ -1,7 +1,7 @@
 import java.util.ArrayList
 
-fun ArrayList<String>.get(index1: Int, index2: Int) = this[index1 + index2]
-fun ArrayList<String>.set(index1: Int, index2: Int, elem: String) {
+operator fun ArrayList<String>.get(index1: Int, index2: Int) = this[index1 + index2]
+operator fun ArrayList<String>.set(index1: Int, index2: Int, elem: String) {
     this[index1 + index2] = elem
 }