#20951 Direct STDERR output when listing crontab to /dev/null

Fix unit test failures

Author: ihor-sviziev

lib/internal/Magento/Framework/Crontab/Test/Unit/CrontabManagerTest.php

@@ -58,7 +58,7 @@ public function testGetTasksNoCrontab()
 
         $this->shellMock->expects($this->once())
             ->method('execute')
-            ->with('crontab -l', [])
+            ->with('crontab -l 2>/dev/null', [])
             ->willThrowException($localizedException);
 
         $this->assertEquals([], $this->crontabManager->getTasks());
@@ -74,7 +74,7 @@ public function testGetTasks($content, $tasks)
     {
         $this->shellMock->expects($this->once())
             ->method('execute')
-            ->with('crontab -l', [])
+            ->with('crontab -l 2>/dev/null', [])
             ->willReturn($content);
 
         $this->assertEquals($tasks, $this->crontabManager->getTasks());
@@ -127,7 +127,7 @@ public function testRemoveTasksWithException()
 
         $this->shellMock->expects($this->at(0))
             ->method('execute')
-            ->with('crontab -l', [])
+            ->with('crontab -l 2>/dev/null', [])
             ->willReturn('');
 
         $this->shellMock->expects($this->at(1))
@@ -148,7 +148,7 @@ public function testRemoveTasks($contentBefore, $contentAfter)
     {
         $this->shellMock->expects($this->at(0))
             ->method('execute')
-            ->with('crontab -l', [])
+            ->with('crontab -l 2>/dev/null', [])
             ->willReturn($contentBefore);
 
         $this->shellMock->expects($this->at(1))
@@ -276,7 +276,7 @@ public function testSaveTasks($tasks, $content, $contentToSave)
 
         $this->shellMock->expects($this->at(0))
             ->method('execute')
-            ->with('crontab -l', [])
+            ->with('crontab -l 2>/dev/null', [])
             ->willReturn($content);
 
         $this->shellMock->expects($this->at(1))