Make BGP state machine use short timers in tests.

Few tests such as rarget_peer_test used to fail occasionally because
bgp peers could not be established within the wait time of the test.

Make all such tests use the StateMachineTest class, which has short
timers. Remove duplicate definitions of StateMachineTest and move it
to bgp_server_test_util.h.

Rename StateMachineTest in state_machine_test to StateMachineUnitTest.

Remove unnecessary use of mock classes for BgpXmppChannelManager and
BgpXmppChannel in a few tests.

Change-Id: I53885053b673da1dcb0725b5d04e9ad00dd94caf