tests: qmp-test: add test for new "x-oob"

tests: qmp-test: add test for new "x-oob"

Test the new OOB capability. It's mostly the reverted OOB test
(see commit 4fd78ad7), but differs in that:

- It uses the new qtest_init_without_qmp_handshake() parameter to
  create the monitor with "x-oob"
- Squashed the capability tests on greeting message
- Don't use qtest_global any more, instead use self-maintained
  QTestState, which is the trend

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180326063901.27425-9-peterx@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: rebase to qtest_init changes]
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Eric Blake <eblake@redhat.com>