phpspec/prophecy is now explicitely needed for tests

[DavidPrevot]

Hi,

Since PHPUnit dropped phpspec/prophecy from its dependencies (sebastianbergmann/phpunit#5033), the following error is triggered when running the testsuite with latest PHPUnit.

There was 1 error:

1) DeepCopyTest\TypeFilter\Spl\ArrayObjectFilterTest::test_it_deep_copies_an_array_object
PHPUnit\Framework\Exception: This test uses TestCase::prophesize(), but phpspec/prophecy is not installed. Please run "composer require --dev phpspec/prophecy".

Regards

David

[mnapoli]

Thanks, a pull request is most welcome if anyone wants to contribute that!