Magento – How to use collection factory model in Unit Test magento 2

I have a custom extension with model with structure like this in database:

+----------------------------------+
|entity_id | customer_id | balance |
+----------------------------------+
|     1    |      3      | 7100000 |
|     2    |      5      | 1250000 |
+----------------------------------+

I usually use my model in __construct function in controller like this:

public function __construct(
.... ,
/Namespace/Module/Model/CreditFactory $credit
... ,
){
  $this->credit = $credit;
}

and i can use to store and retrieve data from my model collection easily like this:

//retrieve Data
$credit = $this->credit->create()->load('1');
$anotherCredit = $this->credit->create()->getCollection()
    ->addFieldToFilter('customer_id','5')->getFirstItem();
$balance = $credit->getBalance();
$anotherBalance = $anotherCredit->getBalance();
//Store Data
$amount = 1000;
$endCreditBalance = $balance - $amount;
$credit->SetBalance(endCreditBalance);
$credit->save();
$endAnotherCreditBalance = $balance + $amount;
$anotherBalance->SetBalance(endAnotherCreditBalance);
$anotherBalance->save();

I tried to create unit test to compared the saved value with the value i want to store like this:

protected function setUp()
{
 $objectManager = new \Magento\Framework\TestFramework\Unit\Helper\ObjectManager($this);
 $this->customerCredit = $objectManager->getObject(\Namespace\Module\Model\CreditFactory::class);      
}

public function testSend()
{
 $credit = $this->credit->create()->load('1');
 $anotherCredit = $this->credit->create()->load('2');

 $balance = $credit->getBalance();
 $anotherBalance = $anotherCredit->getBalance(); 
 $amount = 250;
 $endCreditBalance = $balance - $amount;
 $credit->SetBalance(endCreditBalance);
 $credit->save();
 $endAnotherCreditBalance = $balance + $amount;
 $anotherBalance->SetBalance(endAnotherCreditBalance);
 $anotherBalance->save();
 $result = array(
          'credit_balance' => $credit->getBalance(),
          'another_credit_balance' => $anotherCredit->getBalance()
 );
 $expected = array(
          'credit_balance' => $endCreditBalance,
          'another_credit_balance' => $endAnotherCreditBalance
 );
 $this->assertEquals($expected, $result);
}

but i always got an error like this:

Error: Call to a member function load() on null

and i don't know if the rest of the code will work just fine or not