Add more tests to bgp_xmpp_rtarget_test

New tests cover multiple RTargetRoutes for a RT and path selection.

Change-Id: I25d01765049da9e5d82e07f82e2f7c9e59c86cfe
Add ebgp tests to bgp_xmpp_rtarget_test

Change-Id: Iab8a2549466a2c19b3f5ee1a54b01b7bd89fae6b