--debug-stacktraces causes test failures on Deferred's tests

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From the output of 'trial --debug-stacktraces twisted.test.test_defer':

===============================================================================
[FAIL]: twisted.test.test_defer.LogTestCase.test_errorLog

Traceback (most recent call last):
  File ".../twisted/test/test_defer.py", line 679, in test_errorLog
    self._check()
  File ".../twisted/test/test_defer.py", line 667, in _check
    self.assertEquals(len(c2), 2)
twisted.trial.unittest.FailTest: not equal:
a = 3
b = 2

===============================================================================
[FAIL]: twisted.test.test_defer.LogTestCase.test_errorLogWithInnerFrameCycle

Traceback (most recent call last):
  File ".../twisted/test/test_defer.py", line 706, in test_errorLogWithInnerFrameCycle
    self._check()
  File ".../twisted/test/test_defer.py", line 667, in _check
    self.assertEquals(len(c2), 2)
twisted.trial.unittest.FailTest: not equal:
a = 3
b = 2

===============================================================================
[FAIL]: twisted.test.test_defer.LogTestCase.test_errorLogWithInnerFrameRef

Traceback (most recent call last):
  File ".../twisted/test/test_defer.py", line 692, in test_errorLogWithInnerFrameRef
    self._check()
  File ".../twisted/test/test_defer.py", line 667, in _check
    self.assertEquals(len(c2), 2)
twisted.trial.unittest.FailTest: not equal:
a = 3
b = 2

===============================================================================
[ERROR]: twisted.test.test_defer.LogTestCase.test_errorLog

Traceback (most recent call last):
  File ".../twisted/test/test_defer.py", line 677, in <lambda>
    defer.Deferred().addCallback(lambda x: 1/0).callback(1)
exceptions.ZeroDivisionError: integer division or modulo by zero
===============================================================================
[ERROR]: twisted.test.test_defer.LogTestCase.test_errorLogWithInnerFrameCycle

Traceback (most recent call last):
  File ".../twisted/test/test_defer.py", line 700, in <lambda>
    d.addCallback(lambda x, d=d: 1/0)
exceptions.ZeroDivisionError: integer division or modulo by zero
===============================================================================
[ERROR]: twisted.test.test_defer.LogTestCase.test_errorLogWithInnerFrameRef

Traceback (most recent call last):
  File ".../twisted/test/test_defer.py", line 687, in <lambda>
    d.addCallback(lambda x: 1/0)
exceptions.ZeroDivisionError: integer division or modulo by zero
-------------------------------------------------------------------------------
Ran 53 tests in 0.532s

FAILED (failures=3, errors=3, successes=50)