Fix exercises on objects

[yakutovicha]

fixes #111

[edoardob90]

Don't know (yet) if we want to fix this issue before the next tutorial, but here's something we can do without changing anything substantially: add a test on the type (e.g. isinstance) of the return value of the solution function instead of only its content.

At the moment, we are only testing the content with test_solution = [string for _, string in function_to_test(flavors)]