How to mock a Context object

You can test your class like this:

public function testSomething() 
{
    $contextMock = $this->getMockBuilder(\Magento\Framework\App\Helper\Context::class)
         ->disableOriginalConstructor()
         ->getMock();
    $productImageFactoryMock = $this->getMockBuilder(\Magento\Catalog\Model\Product\ImageFactory::class)
         ->disableOriginalConstructor()
         ->getMock();
    $assetRepoMock = $this->getMockBuilder(\Magento\Framework\View\Asset\Repository::class)
         ->disableOriginalConstructor()
         ->getMock();
    $viewConfigMock = $this->getMockBuilder(\Magento\Framework\View\ConfigInterface::class)
         ->disableOriginalConstructor()
         ->getMock();
    $productRepoMock = $this->getMockBuilder(\Magento\Catalog\Model\ProductRepository::class)
         ->disableOriginalConstructor()
         ->getMock();

    $productImageMock = $this->getMockBuilder(\Magento\Catalog\Model\Product\Image::class)
         ->disableOriginalConstructor()
         ->getMock();

    //mock here any methods you need to mock

    $productImageFactoryMock->method('create')->willReturn($productImageMock); 

    $model = new \The\Class\You\Want\To\Test( 
               $contextMock,
               $productImageFactoryMock,
               $assetRepoMock,
               $viewConfigMock,
               $productRepoMock
    ); 
    //do asserts here
}

You can move all the mock building to the setUp() method if you need to do more than 1 test.

The following code will create a mock of the context in your test.

    $contextMock = $this->getMockBuilder(\Magento\Framework\App\Helper\Context::class)
        ->disableOriginalConstructor()
        ->getMock();

From: vendor/magento/module-sales/Test/Unit/Helper/DataTest.php:setup()

You then will need to use the pass the $contextMock via the the object manager:

$objectManager = new \Magento\Framework\TestFramework\Unit\Helper\ObjectManager($this);

$objectToTest = $objectManager->getObject(
        \Magento\Class\To\Test::class, [
            'context' => $contextMock,
            ...
        ]
    );

Or you can use a Mock and pass the argument if you need need to change the method output of the class you are changing:

$objectToTest = $this->getMockBuilder(\Magento\Class\To\Test::class)
    ->setConstructorArgs([
        'context' => $contextMock
    ])->getMock();