Issue 34900: unittest subTests() fails when called from debug()

Consider this code:

    import unittest


    class TC(unittest.TestCase):

        def test_subtest(self):
            with self.subTest():
                pass


    tc = TC('test_subtest')
    tc.run()

This works when executed, but if we change ``tc.run()`` to ``tc.debug()`` we get the following exception:    

    Traceback (most recent call last):
      File ".tmp\test-unittest-regression.py", line 13, in <module>
        tc.debug()
      File "C:\Users\Bruno\AppData\Local\Programs\Python\Python36\lib\unittest\case.py", line 658, in debug
        getattr(self, self._testMethodName)()
      File ".tmp\test-unittest-regression.py", line 7, in test_subtest
        with self.subTest():
      File "C:\Users\Bruno\AppData\Local\Programs\Python\Python36\lib\contextlib.py", line 81, in __enter__
        return next(self.gen)
      File "C:\Users\Bruno\AppData\Local\Programs\Python\Python36\lib\unittest\case.py", line 512, in subTest
        if not self._outcome.result_supports_subtests:
    AttributeError: 'NoneType' object has no attribute 'result_supports_subtests'    

Looking at the code, ``subTest`` assumes that the ``TestCase`` instance has the ``self._outcome`` atribute, which is set only by ``run()``.

PR 9707 looks good to me. I will merge it once your CLA is processed by the PSF staff. Thanks!

Great, thanks again for the quick review.

FYI, I just noticed that this is a duplicate of issue 29551, but decided to keep this one open.

> FYI, I just noticed that this is a duplicate of issue 29551, but decided to keep this one open.

OK thanks. I did try to search for open issues before filing this one by using the "subtest result_supports_subtests" terms, as I figured a bug report would include the traceback, that's why I didn't find 29551, heh.

New changeset da2bf9f66d0c95b988c5d87646d168f65499b316 by Berker Peksag (Bruno Oliveira) in branch 'master':
bpo-34900: Make TestCase.debug() work with subtests (GH-9707)
https://github.com/python/cpython/commit/da2bf9f66d0c95b988c5d87646d168f65499b316

New changeset 7a98e302c37781f9c6448a28bc70bff18b7e2862 by Berker Peksag (Miss Islington (bot)) in branch '3.7':
bpo-34900: Make TestCase.debug() work with subtests (GH-9707)
https://github.com/python/cpython/commit/7a98e302c37781f9c6448a28bc70bff18b7e2862

New changeset c1fe49c01f3850aaa32a7d75e47f90eb5c5f7efe by Berker Peksag (Miss Islington (bot)) in branch '3.6':
bpo-34900: Make TestCase.debug() work with subtests (GH-9707)
https://github.com/python/cpython/commit/c1fe49c01f3850aaa32a7d75e47f90eb5c5f7efe

messages: + msg327115

type: crash -> behavior