test/msgr: silence warnings from -Wsign-compare

test/msgr: silence warnings from -Wsign-compare

The following warning appears during make:

ceph/src/test/msgr/test_msgr.cc: In member function ‘virtual void MessengerTest_MarkdownTest_Test::TestBody()’:
ceph/src/test/msgr/test_msgr.cc:1474:46: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     CHECK_AND_WAIT_TRUE(srv_dispatcher.count > last + 1);
                         ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~
ceph/src/test/msgr/test_msgr.cc:54:9: note: in definition of macro ‘CHECK_AND_WAIT_TRUE’
     if (expr)                           \
         ^~~~
ceph/src/test/msgr/test_msgr.cc:1475:30: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     if (srv_dispatcher.count == last) {
         ~~~~~~~~~~~~~~~~~~~~~^~~~~~~

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