bzrlib.tests.blackbox.test_serve.TestBzrServe.test_server_exception_with_hook is fragile

This just failed for me on PQM while trying to submit something unrelated.
======================================================================
FAIL: bzrlib.tests.blackbox.test_serve.TestBzrServe.test_server_exception_with_hook (subunit.RemotedTestCase)
bzrlib.tests.blackbox.test_serve.TestBzrServe.test_server_exception_with_hook
----------------------------------------------------------------------
_StringException: Text attachment: log
------------
226.218 creating config directory: '/tmp/testbzr-z3PLaD.tmp/bzrlib.tests.blackbox.test_serve.TestBzrServe.test_server_exception_with_hook/home/.bazaar'
226.220 run bzr: ['serve']
226.220 bazaar version: 2.4.0dev4
226.220 bzr arguments: [u'serve']
226.236 encoding stdout as sys.stdout encoding 'UTF-8'
    INFO catching exception
226.237 errors:
'catching exception\n'
226.260 opening working tree '/tmp/testbzr-z3PLaD.tmp'
------------
Text attachment: traceback
------------
Traceback (most recent call last):
  File "/usr/lib/python2.6/dist-packages/testtools/runtest.py", line 144, in _run_user
    return fn(*args)
  File "/usr/lib/python2.6/dist-packages/testtools/testcase.py", line 465, in _run_test_method
    testMethod()
  File "/home/pqm/bzr-pqm-workdir/home/+trunk/bzrlib/tests/blackbox/test_serve.py", line 107, in test_server_exception_with_hook
    self.assertEqual('listening on port: 4155\ncatching exception\n', err)
AssertionError: not equal:
a = 'listening on port: 4155\ncatching exception\n'
b = 'catching exception\n'

Note that it got the exception before it started listening on the port. I think the fix is that the test should wait until it sees "listening on port" before it triggers the exception.

Either that, or it is a simple stdout/stderr buffering issue. Depending on when the listening line gets flushed, it can not be written before the exception occurs.

Marking this critical because it will cause bouncing from the PQM for things that aren't otherwise broken.