Provide a RoR-like test fixture helper

Any unit test may call $this->loadAllFixtures() from the setUp() routine,
and that will load all fixtures from the civicrm/tests/fixtures directory.

Fixtures can be written using the legacy XML format, or using YAML.
Schema and migration to make Case Type a first-class table

CRM-14473
Case Type refactoring

TODO:
* Test code paths not covered by units.

CRM-14473: Migrate case-types from OptionGroup to new table