test: fix Session mocking

[jamadeo]

Before, the session wasn't mocked correctly. The tests might still pass if the real Session instantiates correctly, but if (like me) you had a global config that pointed to a not-installed plugin, you'd see that failure in these unit tests.

To reproduce the broken bit, if everything was working for you, just add provider: does-not-exist to your ~/.config/goose/profiles.yaml and run this test on main.