Fix documentation for FormTestBase::getMockForm form id parameter

Problem/Motivation

The phpdoc block for the $form_id argument suggests that this method argument is optional, but the method signature does not have an argument value for $form_id.

/**
  * Provides a mocked form object.
  *
  * @param string $form_id
  *   (optional) The form ID to be used. If none is provided, the form will be
  *   set with no expectation about getFormId().

protected function getMockForm($form_id, $expected_form = NULL, $count = 1) {

Proposed resolution

1. Change the API documentation to clarify that the method argument is not optional and should be provided.
2. Change the method signature so that the $form_id argument has a NULL value to match the API documentation.

The only core test class that uses Drupal's getMockForm is \Drupal\Tests\Core\Form\FormBuilderTest. However this probably a useful abstract class for contributed modules.

Remaining tasks

• Choose the approach.
• Add a beta evaluation template.
• Write a patch.
• Review the patch following documentation and code standards.

User interface changes

No.

API changes

Maybe. The method signature either needs to match the documentation or vice versa.

Data model changes

No.

Beta phase evaluation