Fix BuiltinUniqueConstraintWorksWithTargetProxiesConstraintsSpec

[puneetbehl]

PR #1285 introduces a test in tck i.e. BuiltinUniqueConstraintWorksWithTargetProxiesConstraintsSpec which breaks in gorm-hibernate5 because there is no IdentityGenerator with name snowflake.

Following is the exception:

Invalid id generation strategy for entity [grails.gorm.tests.Root]: snowflake
org.grails.datastore.mapping.model.DatastoreConfigurationException: Invalid id generation strategy for entity [grails.gorm.tests.Root]: snowflake
	at org.grails.orm.hibernate.cfg.HibernateMappingContext$HibernateMappingFactory.createIdentityMapping(HibernateMappingContext.java:246)
	at org.grails.datastore.mapping.model.config.GormMappingConfigurationStrategy.getIdentityMapping(GormMappingConfigurationStrategy.java:928)
	at org.grails.datastore.mapping.model.config.JpaMappingConfigurationStrategy.getIdentityMapping(JpaMappingConfigurationStrategy.java:289)
	at org.grails.datastore.mapping.model.AbstractClassMapping.<init>(AbstractClassMapping.java:35)
	at org.grails.orm.hibernate.cfg.HibernatePersistentEntity$1.<init>(HibernatePersistentEntity.java:35)
	at org.grails.orm.hibernate.cfg.HibernatePersistentEntity.<init>(HibernatePersistentEntity.java:35)
	at org.grails.orm.hibernate.cfg.HibernateMappingContext.createPersistentEntity(HibernateMappingContext.java:108)
	at org.grails.datastore.mapping.model.AbstractMappingContext.addPersistentEntities(AbstractMappingContext.java:274)
	at org.grails.orm.hibernate.cfg.HibernateMappingContext.<init>(HibernateMappingContext.java:73)
	at org.grails.orm.hibernate.connections.HibernateConnectionSourceFactory.buildConfiguration(HibernateConnectionSourceFactory.java:94)
	at org.grails.orm.hibernate.connections.HibernateConnectionSourceFactory.create(HibernateConnectionSourceFactory.java:85)
	at org.grails.orm.hibernate.connections.AbstractHibernateConnectionSourceFactory.create(AbstractHibernateConnectionSourceFactory.java:39)
	at org.grails.orm.hibernate.connections.AbstractHibernateConnectionSourceFactory.create(AbstractHibernateConnectionSourceFactory.java:23)
	at org.grails.datastore.mapping.core.connections.AbstractConnectionSourceFactory.create(AbstractConnectionSourceFactory.java:64)
	at org.grails.datastore.mapping.core.connections.AbstractConnectionSourceFactory.create(AbstractConnectionSourceFactory.java:52)
	at org.grails.datastore.mapping.core.connections.ConnectionSourcesInitializer.create(ConnectionSourcesInitializer.groovy:24)
	at org.grails.orm.hibernate.HibernateDatastore.<init>(HibernateDatastore.java:222)
	at org.grails.orm.hibernate.HibernateDatastore.<init>(HibernateDatastore.java:276)
	at org.grails.datastore.gorm.Setup.setup(Setup.groovy:80)
	at org.grails.datastore.gorm.Setup.setup(Setup.groovy:73)
	at grails.gorm.tests.GormDatastoreSpec.createSession(GormDatastoreSpec.groovy:47)
	at grails.gorm.tests.GormDatastoreSpec.setup(GormDatastoreSpec.groovy:42)