Fixed dynamic admin inlines for subclasses doing fields magic
(assuming that InlineAdmin.fields is a user-provided list).
Added a test for DynamicInlineAdmin.
Allow DynamicInlineAdmin fields to be a tuple.

Previously, if fields was defined as a tuple, a very confusing TypeError
would be raised.
Shamelessly modified the dynamic admin fields tuple test, so it checks
get_fieldsets() instead of directly using the fields attribute.

Also made BaseDynamicInlineAdmin work when fields are listed without
the _order field or fieldsets are declared (with or without it).