Change Aggregate test for Event get_one() and get_all()

[vladyslav-fenchak]

Change tests and call get_attr() and get_attrs().
Related to #195

[garciparedes]

Hi @andrea-mucci and @vladyslav-fenchak!

I think get_field and get_fields is a better choice as we use the word Field instead of Attr for the field class (as you can see at minos.aggregate.Field).

Could you consider these names instead of get_attr and get_attrs?

[andrea-mucci]

Hi @garciparedes, have sense, ok i will discuss with @vladyslav-fenchak to modify again