Document how to make views unit tests

Problem/Motivation

While I was trying to figure out #1791970: Optionally detect tar.* as extension instead of * on views file extension field handler I noticed the procedure to make views unit test is a little tricky, and some maintainers agreed with me about that.

So, what I think we should be documented:

• It is recommmended to use a custom module to test views handlers.
• Those modules should live on module's tests/modules directory.
• It is recommmended to define new views instead of reusing the ones provided by views and change them at runtime, unless there is a good reason behind that.
• To have the defined view loaded you need to:
  • add the view machine name on the $testViews data member declaration
  • store the exported view on test module's test_views directory with file name views.view.<machine_name>.yml
  • Implement setUp and call ViewTestData::importTestViews() passing the module containing the default test view.
• It is recommended define the test calss on Drupal\file\Tests\Views namespace.
• Mention how to use dataSet() and viewsData().
• Maybe an basic example (an extension of Drupal\views\Tests\BasicTest?).

Proposed resolution

Let's document it!

Remaining tasks

• Decide where to document it. Current propposals:
  • at Drupal\views\Tests\ViewUnitTestBase
  • at Drupal\views\Tests\BasicTest

User interface changes

None.

API changes

None.

Related Issues

• #1856544: [META] Views documentation improvements