Update tests to detect duplicate movies in get_similar_movies.
Fix duplicate output bugs in transitive movie lookup method.

There were two bugs causing duplicate movies to appear:
1. Failure to check if a movie was related to two movies
that are both related to the same other movie.
2. Mutable python object in default parameter value.

This commit fixes both with a refactor that has the benefit
of also simplifying method logic.
Use Counter for similar movie comparisons in tests.

This commit updates tests to use collections.Counter
when testing the transitive movie lookup method. The
new code validates that the correct movies are present
in the list and that there are no duplicates, but doesn’t
care about the order movies appear in the list.
Fix names in tests to match model method.

Should have been done in 256b7de.
Setup test environment using objects instead of object attributes.