Drupal - How to mock a node field value for phpunit?

The following should work using PHPUnit Mock Objects as in your example.

First mock FieldItemListInterface __get magic method.

$fieldDescMock = $this->getMockBuilder('\Drupal\Core\Field\FieldItemListInterface')
  ->disableOriginalConstructor()
  ->getMock();
$fieldDescMock->expects($this->any())
  ->method('__get')
  ->with('value')
  ->willReturn('blah');

Then mock the Node/ContentEntityBase __get magic method.

$node->expects($this->any())
  ->method('__get')
  ->with('field_description')
  ->willReturn($fieldDescMock);

Mradcliffe's answer worked for me, here's my example code:

Here's what ended up working for me:

$this->node = $this->getMockBuilder(Node::class)
  ->disableOriginalConstructor()
  ->getMock();

$fieldAdsEnabled = $this->getMockBuilder(FieldItemListInterface::class)
  ->disableOriginalConstructor()
  ->getMock();
$fieldAdsEnabled->expects($this->any())
  ->method('__get')
  ->with('value')
  ->willReturn(1);
$this->node->expects($this->any())
  ->method('__get')
  ->with('field_widgets_enabed')
  ->willReturn($fieldAdsEnabled);

Note: for me using Node object and not NodeInterface helped. I suspect because otherwise __get isn't available.