test/mon: silence warnings from -Wreorder

test/mon: silence warnings from -Wreorder

The following warning appears during make:

ceph/src/test/mon/test-mon-msg.cc: In constructor ‘MonMsgTest::MonMsgTest()’:
ceph/src/test/mon/test-mon-msg.cc:222:12: warning: ‘MonMsgTest::reply_msg’ will be initialized after [-Wreorder]
   Message *reply_msg;
            ^~~~~~~~~
ceph/src/test/mon/test-mon-msg.cc:229:16: warning:   base ‘MonClientHelper’ [-Wreorder]
     lock("lock") { }
                ^
ceph/src/test/mon/test-mon-msg.cc:226:3: warning:   when initialized here [-Wreorder]
   MonMsgTest() :

Signed-off-by: Jos Collin <jcollin@redhat.com>