unittest2 recently added the ability to show local variables in
tracebacks as https://github.com/testing-cabal/testtools/issues/111
requested for us. Reusing that requires some refactoring of our code,
in particular where we were reimplementing bits of the traceback
module. Now we can just hard-depend on traceback2 and linecache2 which
are brought in by unittest2 1.0.0.

Change-Id: Ieb3268029d26b48ed4fcd25ed644bd339f6aa3fb

``testtools.TestCase`` now inherits from unittest2.TestCase, which
provides a ``setUpClass`` for upcalls on Python 2.6.
(Robert Collins, #1393283)

Change-Id: Id56212e3d7d519c7b73d2e19d3e34013fac34544

Change-Id: I1f314c91c4e852edafabc7cf3d2b01abe8a62fa3

Exceptions in a ``fixture.getDetails`` method will no longer mask
errors raised from the same fixture's ``setUp`` method.
(Robert Collins, #1368440)

Change-Id: I39da334ba57683fd71a5d7af16c32d9170f6e626

Fix a long-standing bug where tearDown and cleanUps would not be
called if the test run was interrupted. This should fix leaking
external resources from interrupted tests.
(Robert Collins, #1364188)

Fix a long-standing bug where calling sys.exit(0) from within a test
would cause the test suite to exit with 0, without reporting a failure
of that test. We still allow the test suite to be exited (since
catching higher order exceptions requires exceptional circumstances)
but we now call a last-resort handler on the TestCase, resulting in an
error being reported for the test.
(Robert Collins, #1364188)

Change-Id: I0700f33fe7ed01416b37c21eb3f3fd0a7ea917eb

TestCase.setUp and TestCase.tearDown are only meant to be called once.

Change-Id: If39046eed35257938062b07655b50d9bef0ea5ee
Closes-Bug: #882884

Fix expected failure