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

compiler/testData/codegen/box/functions/invoke/kt3821invokeOnThis.kt

@@ -1,7 +1,7 @@
 //KT-3821 Invoke convention doesn't work for `this`
 
 class A() {
-    fun invoke() = 42
+    operator fun invoke() = 42
     fun foo() = this() // Expecting a function type, but found A
 }