MNT Don't use magic number in unit test

tests/php/Forms/FormFieldTest.php

@@ -983,7 +983,7 @@ public function testFieldValidatorConfig(): void
         }
         // Assert that we have tested all classes e.g. namespace wasn't changed, no new classes were added
         // that haven't been tested
-        $this->assertSame(58, $count);
+        $this->assertSame(count($expectedFieldValidators), $count);
     }
 
     private function instantiateFormField(string $class): FormField