Add tests for global categories, releases, fix bugged tests

[kevinsekuj]

Additions

• Added further unit tests for new releases, passing limit parameter with minimum and maximum values of 1 and 50
• Added further unit tests for categories, omitting country code to test global releases

Fixes

• Fixed potential error where found variable in test_artist_related_artists tests/integration/non_user_endpoints/test.py is undefined if for loop never evaluates to true
• Fixed false positive test test_new_releases which looks up the wrong property of the JSON response object and always evaluates to true

[kevinsekuj]

@stephanebruckert any ideas why the job for python build fails?

[stephanebruckert]

@kevinsekuj I just fixed it on the default branch

[stephanebruckert]

Thanks!