Many unit tests can only be run if the version of Django includes

certain support. For instance, migration-related unit tests require

support for migrations in Django. Previously, every such unit test had

to have its own checks and handle raising a SkipTest, but this was

getting tedious to maintain, and isn't very future-proof.
This change adds some new decorators for these tests, and standardizes

the SkipTest message. This simplifies much of the code and improves

maintainability going forward.

All unit tests pass across all versions of Django.
Verified that the skip message output was correct in older versions.