validate_logging interacts badly with trial-based, Deferred-returning tests that fail with SkipTest

[exarkun]

This is like #153 but for the case where the test uses Twisted, returns a Deferred, and the Deferred fires with SkipTest:

from eliot.testing import assertHasAction, validate_logging
from eliot import Action

from twisted.trial.unittest import SkipTest, TestCase
from twisted.internet.defer import fail

class LogTests(TestCase):
    @validate_logging(
        lambda case, logger:
            assertHasAction(case, logger, Action, succeeded=True)
    )
    def test_log(self, logger):
        return fail(SkipTest("Oops"))

$ trial it.py 
it
  LogTests
    test_log ...                                                      [SKIPPED]
                                                       [ERROR]

===============================================================================
[SKIPPED]
Oops

it.LogTests.test_log
===============================================================================
[ERROR]
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 139, in maybeDeferred
    result = f(*args, **kw)
  File "/usr/lib/python2.7/dist-packages/twisted/trial/_asynctest.py", line 273, in <lambda>
    return lambda: f(*args, **kwargs)
  File "/home/jean-paul/Projects/eliot/master/eliot/testing.py", line 270, in <lambda>
    self, logger, *assertionArgs, **assertionKwargs))
  File "/home/jean-paul/Projects/eliot/master/it.py", line 8, in <lambda>
    @validate_logging(lambda case, logger: assertHasAction(case, logger, Action, succeeded=True))
  File "/home/jean-paul/Projects/eliot/master/eliot/testing.py", line 354, in assertHasAction
    testCase.assertTrue(actions, "No actions of type %s" % (actionType,))
  File "/usr/lib/python2.7/dist-packages/twisted/trial/_synctest.py", line 308, in assertTrue
    raise self.failureException(msg)
twisted.trial.unittest.FailTest: No actions of type <class 'eliot._action.Action'>

it.LogTests.test_log
-------------------------------------------------------------------------------
Ran 1 tests in 0.013s

FAILED (skips=1, errors=1)