Add tests for existing extensions

[vhenzl]

Unit tests for DoctrineSelectableClassReflectionExtension, DoctrineSelectableDynamicReturnTypeExtension and EntityManagerFindDynamicReturnTypeExtension.

[ondrejmirtes]

Thanks a lot, I really appreciate this! I merged this into master with some changes:

1. I had to update some tests because getReference/find/etc. now return mixed by default. It means that anything can be called on it and that PHPStan does not report "Cannot call method X() on object."
2. I like asserting type descriptions (strings) rather than asserting instances of Type objects. I consider them an implementation detail, but string type descriptions are considered public-facing.
3. I added a commit that integrates the new phpstan-phpunit extension which correctly handles mocks: 6ad0f3d