tests: Add check-qnull

Add a new test, for checking reference counting of qnull(). As
part of the new file, move a previous reference counting change
added in commit a861564 to a more logical place.

Note that while most of the check-q*.c leave visitor stuff to
the test-qmp-*-visitor.c, in this case we actually want the
visitor tests in our new file because we are validating the
reference count of qnull_, which is an internal detail that
test-qmp-*-visitor should not be peeking into (or put another
way, qnull() is the only special case where we don't have
independent allocation of a QObject, so none of the other
visitor tests require the layering violation present in this
test).

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1461879932-9020-14-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
