Migrate clustering subsystem tests to use AbstractSubsystemSchemaTest from wildfly-core

Clustering subsystems tests already extend org.jboss.as.clustering.subsystem.ClusteringSubsystemTest, which was migrated to wildfly-core as org.jboss.as.subsystem.test.AbstractSubsystemSchemaTest.
So, for the following subsystems:

• Infinispan
• JGroups
• mod_cluster
• singleton
• distributable-ejb
• distributable-web

... we just need to swap super classes (and maybe some minor pom cleanup).