truncate_plan should truncate each app separately

# Context I have a project with multiple apps (Ex: App A and app B). I once in a while code migrations with tests based on the state of the DB at that time. Lately I added a new `nullable` field on my Old model in app A, a migration to fill the field then make it `non-nullable`. Everything was working properly except one thing, an old migration test started to fail. The old migration tests for app B started to fail because back then that field did not exist but my app A was being fully migrated. It did not make any sense until I realized `truncate_plan` only truncate the end of the migrations plan and not the apps separately. # Explanations Lets say I have migrations in each apps that were added in that chronological order: ```python A1 B1 A2 B2 A3 B3 (has explicit dependency to A3 here) A4 B4 A5 B5 (this is a custom migration) B6 A6 (Add new nullable field here) A7 (Migration to fill the field in the DB) A8 (Migration to make field non-nullable) ``` The migration test is specified like this: ```python migration_from = [ ("A", "4"), ("B", "4"), ] migrate_to = [ ("B", "5"), ] ``` However in practice when the test is ran the full plan looks like this: ```python A1 A2 A3 A4 A5 A6 (Add new nullable field here) A7 (Migration to fill the field in the DB) A8 (Migration to make field non-nullable) B1 B2 B3 (has explicit dependency to A3 here) B4 B5 (this is a custom migration) B6 ``` The truncate plan A4 B4 will result to start dropping everything after index of B4 with the [current implementation](https://github.com/wemake-services/django-test-migrations/blob/128efac0494185d150df5c940baa0f6204bfc359/django_test_migrations/plan.py#L115-L116): ```python A1 A2 A3 A4 A5 A6 (Add new nullable field here) A7 (Migration to fill the field in the DB) A8 (Migration to make field non-nullable) B1 B2 B3 (has explicit dependency to A3 here) B4 # B5 (this is a custom migration) <- REMOVAL FROM PLAN BEGINS AT THIS INDEX # B6 ``` However I would expect `truncate_plan` A4 B4 to truncate per app independently like this: ```python A1 A2 A3 A4 # A5 <- REMOVED BECAUSE IT IS ABOVE A4 # A6 (Add new nullable field here) <- REMOVED FROM PLAN BECAUSE IT IS ABOVE A4 # A7 (Migration to fill the field in the DB) <- REMOVED FROM PLAN BECAUSE IT IS ABOVE A4 # A8 (Migration to make field non-nullable) <- REMOVED FROM PLAN BECAUSE IT IS ABOVE A4 B1 B2 B3 (has explicit dependency to A3 here) B4 # B5 (this is a custom migration) <- REMOVED FROM PLAN BECAUSE IT IS ABOVE B4 # B6 <- REMOVED FROM PLAN BECAUSE IT IS ABOVE B4 ```