Fix flaky tests further broken by XMPP ASN change fix.

As a result of commit e383a8eedc0b3b7106ea5ed13d5f92ee2753ade0 all XMPP
connections are flapped when the ASN is changed.  Since the mock agent
used in some unit/module tests does not re-subscribe to instances if a
connection flaps, some tests got broken.

Specifically, tests that did not configure an ASN (or anything else) at
the start of the test, brought up XMPP connections to mock agents, and
only then configured the ASN (and routing-instances), were impacted.

Fix is to tweak the tests to minimally configure the ASN at the start.

Also tweak the ASNUpdate test in rtarget_peer_test to account for XMPP
connection flap.

Use XmppStateMachineTest which uses small timer values so that the XMPP
connections come back quickly after a flap. Add the class in the common
bgp_server_test_util.h file and remove duplicate definitions from other
places.

Change-Id: I67f739c8f1fcdda4ac923fa42d74e75b343180ae
Merge "Fix flaky tests further broken by XMPP ASN change fix."