Unit tests for schema changes

Useful unit tests:

• CRM_Volunteer_BAO_Assignment::createVolunteerActivity() - test that the volunteer beneficiary is made the activity target
• CRM_Volunteer_BAO_Assignment::createVolunteerActivity() - test that the activity subject defaults to the project title
• civicrm_api3('VolunteerProject', 'create') - test the new project_contacts parameter
• civicrm_api3('VolunteerProjectContact', 'create')
• civicrm_api3('VolunteerProjectContact', 'get')
• civicrm_api3('VolunteerProjectContact', 'delete')