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

compiler/testData/codegen/box/classes/overloadPlusToPlusAssign.kt

@@ -7,14 +7,14 @@ class ArrayWrapper<T>() {
         contents.add(item)
     }
 
-    fun plus(rhs: ArrayWrapper<T>): ArrayWrapper<T> {
+    operator fun plus(rhs: ArrayWrapper<T>): ArrayWrapper<T> {
         val result = ArrayWrapper<T>()
         result.contents.addAll(contents)
         result.contents.addAll(rhs.contents)
         return result
     }
 
-    fun get(index: Int): T {
+    operator fun get(index: Int): T {
         return contents.get(index)!!
     }
 }