php unit testing - Property does not exist

I am creating some test cases for my application and i am stuck at a point. I am trying to set a property to the class. But its a parent Class property. This is my actual method

public function uploadFiles($files, $endpoint = 'outbox', $config = [])
    {
        $user = $this->_BxApi->users->getUser($config);
        $result = [];
        foreach ($files as $key => $file) {
            $policy = 'policy' . $key;
            $result[] = $this->_fileUploader->uploadFile($file, $policy, $config);
        }

        return $result;
    }

This is a method in Class Uploads which extent Class API and the API class is

class Api
{

    public function __construct(BxClient $bxclient)
    {
        $this->_BxApi = $bxclient;
    }
}

And my test case is

public function testUploadFiles_Upload2Files()
{
    $bxMock = $this->_getMock(\BxClient\Core\Api\Uploads::class, [
        '_request' => true
    ]);

    $this->_setProperty($bxMock, 'users', $this->_getMock(Users::class, [
        'getUser' => (object) ['userId' => 'test']
    ]));
    $this->_setProperty($bxMock, '_fileUploader', $this->_getMock(FileUploader::class, [
        'uploadFile' => true,
    ]));

    $result = $bxMock->uploadFiles(['1.txt', '2.txt']);

    $this->assertEquals(2, count($result));
    $this->assertTrue($result[0]);
    $this->assertTrue($result[1]);
}

I think I am doing something wrong in

$this->_setProperty($bxMock, 'users', $this->_getMock(Users::class, [
        'getUser' => (object) ['userId' => 'test']
    ]));

Thanks in advance.