C++ green 2

Another attempt to get the perch/FreeBSD matrix job passing:
https://ci.openmicroscopy.org/view/Failing/job/OMERO-5.1-merge-cpp-cmake/label=perch/

@rleigh-dundee has pointed out that that VM is particularly slow and so it might be a matter of having to special case the particular failure that we're seeing. What I think is happening is that it's taking so long for the VM to connect to the server and register it's callback that "finished()" is never being called: `ASSERT_EQ(1, finished)`.

Assuming that's the case, then we'll force an extra check. If that's **not** the case, then we may be into some strange locking issue, since testing on the box itself, I previously saw entry and exit of the `finished()` method occurring only **after** the `ASSERT_EQ` had passed. (Another possible lead here is that the `PollThread` itself could be holding onto the `mutex`...)