PHPUnit Testing when a custom class contains drupal calls and references other classes
If you are doing Unit tests (and not functional tests), you will need to mock the objects you want to use, in the setup of the test. You can read more here: https://www.drupal.org/docs/8/phpunit/mocking-entities-and-services-with-phpunit-and-mocks

If you are using Drupal functions, you should first try to find out if there is an OOP method that can be used to do the same thing - later versions of Drupal have tried to deprecate such functions, and provide OOP alternatives instead. An example is the Time (datetime.time) service, that replaces things like REQUEST_TIME.

If there is no OOP alternative, you can do the following. Imagine you are using db_query() (which, you shouldn't). Instead of using that function direct in your code, you can create a protected function as follows:

protected function dbQuery($query, array $args = [], array $options = []) {
  return db_query($query, $array, $options);
}


Next, for your text, you will create a class that extends the original class. In this test class, you override dbQuery() above:

protected function dbQuery($query, array $args = [], array $options = []) {
  return ['row1' =&gt; ['col1' =&gt; 'val1'], 'row2' =&gt; ['col2' =&gt; 'val2']];
}


This mocks a call to db_query by returning an array of values. The last step is to use this new class in your Unit tests, so that when dbQuery() is called, it uses your extended version that returns the array, instead of calling db_query() which is not available to the Unit test.