offloaded: fix failure in unit test on windows

offloaded: fix failure in unit test on windows

in trying to test my fix for the failure of the offloaded unit test on windows
(by closing the reader before unlinking the encoding file - which, perhaps
disturbingly doesn't actually make a difference in my windows environment)
I was unable too because the unit test failed every time with a connection lost
error.

after much more time than I'd like to admit it took, I eventually managed to
track that down to a part of the unit test which is supposed to be be dropping
a connection.   it looks like the exceptions that get thrown on unix, or at
least all the specific environments brian tested in, for that dropped
connection are different from what is thrown on my box (which is running py2.4
and twisted 2.4.0, for reference)  adding ConnectionLost to the list of
expected exceptions makes the test pass.

though curiously still my test logs a NotEnoughWritersError error, and I'm not
currently able to fathom why that exception isn't leading to any overall
failure of the unit test itself.

for general interest, a large part of the time spent trying to track this down
was lost to the state of logging.  I added a whole bunch of logging to try
and track down where the tests were failing, but then spent a bunch of time
searching in vain for that log output.  as far as I can tell at this point
the unit tests are themselves logging to foolscap's log module, but that isn't
being directed anywhere, so all the test's logging is being black holed.