refactor: merge testing round and comparing classes into the main backend testing class

[datapythonista]

The methods of the classes RoundAwayFromZero, RoundHalfToEven and UnorderedComparator can be merged into the Backend class, and then implement an attribute to select which method to use, for example:

class Spark(Backend):
    round_method = 'half_to_even'

This would make things simpler, than the current mixin approach.

[cpcloud]

Sure, PRs welcome.