Stub out code list updates where not important. #3994.

Code list updating is tested in the base class. The other behaviors in
these migrations do not rely on the new code lists. Stubbing them out
provides a substantial spec performance improvement.
2.2 -> 3.1 does not need a custom migration.

If a version migration has no changes other than the code list, using
the Basic migration directly is fine.

#3628; noticed with #3994.
Merge pull request #205 from NUBIC/3994-faster_version_migration_specs

Speed up version migration specs (#3994)