tests: add a comment about our fake qmp

tests: add a comment about our fake qmp

While testing QMP, I used a simple qemu session of
'qemu-kvm -M none -nodefaults -nographic -qmp stdio'
for some experiments.  But it took me far too long to remember
the magic invocation to unlock QMP into accepting normal commands.
While I was able to grep libvirt sources and easily find where
libvirt expects the normal "QMP" greeting, I could not find the
proper reply to that greeting nearby.

Reading the testsuite didn't help either, since there we don't
emulate the mandatory handshake.  But since my grep hit the
testsuite, adding a bit of documentation will make it much easier
to jog my memory in the future.

* tests/qemumonitortestutils.c (QEMU_JSON_GREETING): Mention that
the normal counterpart reply is skipped.