Remove request in test_is_connected that tries to 'wake' up server select loop

This was required with the pre-tornado implementation since the loop had a
fairly long timeout which would cause server.join() to block, and no way of
waking it without causing network io. The tornado based implementation actually
exits cleanly as soon as stop() is called, causing the unnecessary request to
fail. Due to a race condition, it did not happen all the time, but could have
been quite reliably reproduced by adding a time.sleep() before the ?watchdog
request is made.
Add ability for wait_sensor to also test status
Remove race condition in test_bad_client_socket

The KATCPserver._stream_closed_callback is called async with no specific
synchronisation, which is fine, since it only needs to remove the connection
form the KATCPserver._connections dictionary to prevent a memory leak. Since
this test specifically tests that it is removed, we need to wait for the call to
happen. It will definitely get called if we let the ioloop run once, so adding
self.server.sync_with_ioloop() in the test fixes the race.
Merge pull request #25 from ska-sa/minor-test-reliability-fixup

Several test reliability improvements