Fixed failed unit test

Author: engcom-Charlie

app/code/Magento/Indexer/Test/Unit/Controller/Adminhtml/Indexer/MassOnTheFlyTest.php

@@ -223,6 +223,8 @@ public function testExecute($indexerIds, $exception, $expectsExceptionValues)
                 ->willReturn($indexerInterface);
 
             if ($exception !== null) {
+                $indexerInterface->expects($this->any())
+                    ->method('isScheduled')->willReturn(true);
                 $indexerInterface->expects($this->any())
                     ->method('setScheduled')->with(false)->willThrowException($exception);
             } else {