lease-server-test: Destroy server on shutdown

lease-server-test: Destroy server on shutdown

libcheck tests normally run each test in a separate process
so the server objects will automatically be destroyed after
each test.  If the test suite is run with CK_FORK=no, for
example when debugging the test with gdb, failing to
destory the server object will leave the test sockets
from the previous test open, which will stop the next
test from creating its own sockets.

Bug-AGL: SPEC-3816

Change-Id: I42a0976c392bdc19bb60acc78178aac8ee3e14ce
Signed-off-by: Damian Hobson-Garcia <dhobsong@igel.co.jp>