Replace expect()->isInstanceOf() with verify()->instanceOf()

codeception/verify 2.1 removed support for expect()->isInstanceOf() so we need
to replace it with verify()->instanceOf().

[MAILPOET-5664]

mailpoet/tests/unit/API/JSON/ErrorHandlerTest.php

@@ -38,7 +38,7 @@ class ErrorHandlerTest extends \MailPoetUnitTest {
       'key' => 'value',
     ]));
 
-    expect($response)->isInstanceOf(ErrorResponse::class);
+    verify($response)->instanceOf(ErrorResponse::class);
     verify($response->status)->equals($expectedCode);
     verify($response->errors)->equals([['error' => 'key', 'message' => 'value']]);
   }