Fixed #22245 -- Avoided widget overwrite in forms.IntegerField subclasses

Thanks Jeroen Pulles for the report and Simon Charette for the review.
Fixed #6103 -- Splitted tests in model_forms tests
Merged model_forms_regress with model_forms tests
Fixed #22272 -- Fixed regression in DecimalField when using decimal_places=0.

Thanks to trac user merb for the report.