Consider migrating from phpspec/prophecy to PHPUnit mocks for test doubles

[acelaya]

Pros:

• One less dependency.
• Get rid of workarounds, as PHPUnit no longer depends on prophecy.
• Get rid of the use of traits.
• PHPUnit is better maintained (more frequent updates and earlier support for new PHP versions).
• Be able to use phpstan on tests via plugin, to make sure mocks are properly used.

Cons:

• Syntax is slightly less nice.
• Requires a bit more boilerplate to build mocks.
• It's not possible to move expectations to after subject under test is invoked. You need to to predictions instead.

Docs: https://phpunit.readthedocs.io/en/9.5/test-doubles.html