splango.models.Goal: added get_records_total()
splango.tests: removed experiment from enrollment factory

it is not necessary, it was removed from enrollment model too.
splango.tests.test_models.Enrollment: added test unique together
splango.tests.test_models: added Test suffix to avoid ambiguity
between classes from this file and splango.models
splango.tests.test_models: added basic tests for Enrollment
splango.tests.test_models: added tests for Goal.get_records_total and Goal.get_records_count_per_variant