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tests: Prevent “unittest --buffer” from crashing

Before this change, if certain tests were failing in certain ways, then
running “python -m unittest --buffer” would cause an AttributeError in
the unittest module itself.

Here’s what unittest does when you use the --buffer argument:

1. It sets sys.stdout and sys.stderr to StringIOs [1].
2. It runs a test.
3. If the test failed, it runs getvalue() on sys.stdout and sys.stderr
   to get data from its StringIOs.

tests/test_cli.py has its own RunContext class that does something
similar. Before this change, here’s what could happen:

1. unittest sets sys.stdout and sys.stderr to StringIOs.
2. unittest runs a test that uses RunContext.
3. A RunContext gets entered. It sets sys.stdout and sys.stderr to its
   own StringIOs.
4. The RunContext gets exited. It sets sys.stdout and sys.stderr to
   sys.__stdout__ and sys.__stderr__.
5. The test fails.
6. unittest assumes that sys.stdout is still set to one of its
   StringIOs, and runs sys.stdout.getvalue().
7. unittest crashes with this error:

	AttributeError: '_io.TextIOWrapper' object has no attribute 'getvalue'

[1]: <https://github.com/python/cpython/blob/<a class=hub.com/adrienverge/yamllint/commit/2305ca51448552542b2414186252123a8dc87db7">2305ca514/Lib/unittest/result.py#L65>
[2]: <https://github.com/python/cpython/blob/2305ca51448552542b2414186252123a8dc87db7/Lib/unittest/result.py#L87>

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