Add phpstorm metadata for mock methods when testing

[bentleyo]

Summary

This adds some PhpStorm metadata to help with the return type when calling the $this->mock, $this->partialMock etc. methods in tests. By default PhpStorm believes the return type is an object implementing MockInterface, but this PR adds an additional typehint for the mocked class.

Here is an example in a test:

$user = $this->mock(User::class);

PhpStorm will now say the return type is App\Models\User&Mockery\MockInterface.

Type of change

• New feature (non-breaking change which adds functionality)

Checklist

• Add a CHANGELOG.md entry

[mfn]

LGTM!