public function execute(Observer $observer)
    {
        if ($observer->getData('status') == Status::REQUIRED) {
            //implementation
        }
    }

$eventObserverMock = $this->getMockBuilder(Observer::class)
            ->setMethods(['getData'])
            ->disableOriginalConstructor()
            ->getMock();

$eventObserverMock
        ->expects($this->any())
        ->method("getData")
        ->with('status')
        ->willReturn('some value'); // here you can set your value which you want and use `$eventObserverMock` while calling your `execute` method