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


Then mock the Node/ContentEntityBase __get magic method.

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