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

compiler/testData/codegen/boxWithJava/reflection/mutatePrivateJavaInstanceField/K.kt

@@ -5,6 +5,6 @@ fun box(): String {
     val a = J()
     val p = J::class.members.single { it.name == "result" } as KMutableProperty1<J, String>
     p.isAccessible = true
-    p[a] = "OK"
-    return p[a]
+    p.set(a, "OK")
+    return p.get(a)
 }