PHPUnit: Fix slow tests

[obenland]

Takes test execution from 7.8s to ~3s.

class-test-followers.php needs a keen eye to see if the AI has inadvertently removed an implicit test that we should keep.

Once we drop support for PHP 7.0, the listener can be enabled by default.

Proposed changes:

• Adds a timing listener to tests
• Adds http request mocks and breaks up tests to make them faster.

Other information:

• Have you written new tests for your changes, if applicable?

[pfefferle]

I am Fine with catching up with WordPress and require 7.2!

[pfefferle]

Maybe we can use JSON files in a future PR... something like sanitize_title( $host ).json. It would be easier to add new platforms or to update the existing ones, this way.