Tests: investigate alternatives for mocking

[jrfnl]

The functionality within PHPUnit itself to mock classes and set expectations on them is being deprecated left right and center, with each upcoming PHPUnit version needing more work-arounds.

While Requests doesn't use mocking much (at all), it is used in a few select places and keeping that working is getting more difficult and will probably become impossible come PHPUnit 11/12.

Typical issues:

• setMethods() was deprecated in PHPUnit 8.3 and removed in PHPUnit 10.
• The alternative addMethods() was deprecated in PHPUnit 10.1 and is expected to be removed in PHPUnit 12.
• The getMockBuilder() method is deprecated and is expected to be removed in PHPUnit 12.

It should be investigated if using Mockery would be an alternative (once it is available with support for PHPUnit 10).