Moved FixBaseclassesTest tor test_migration_browser.py

This is easier to find  promots cleaner separation.
add PLONE_APP_CONTENTTYPES_MIGRATION_TESTING layer.

This layer only installs Products.ATContenttypes.
It's used to have a proper environment to test migration
to plone.app.contenttypes.
add ATBlob tests and use migration layer for test_migration
cleanup tests: Products.ATContenttypes has no gs profile to run at all
Merge branch 'master' into migration_refactore_tests

Conflicts:
	plone/app/contenttypes/testing.py
	plone/app/contenttypes/tests/test_migration.py
Fix regression: plone.app.contenttypes.interfaces.IEvent moved
cleanup tests