Add TestCase.enter_context to make it easier to use context managers in tests.

When writing tests, re-using the same context manager logic is hard because
of the separatation of setUp() from the test being executed. Re-using the
logic require decorating the test function (which usually adds an argument
to the signature), or copy/pasting the same `with` block (which loses a
level of indent and can create a lot of redundant code).

This can be particularly annoying with mock.patch, since you must either
decorate every function, or add very boilerplate, awkward, and somewhat subtle
cleanup logic in setUp to properly patch, start, store, and cleanup the
patching and its created mock object.

To alleviate this, contextlib.ExitStack can be used to more manually manage
the lifecycle of context managers.

NOTE: This is a Python 3-only feature because contextlib.ExitStack is Py3-only.
While contextlib2 could be used for Py2 support, that involves pulling in another
dependency, so can be done in a future change.
PiperOrigin-RevId: 277084054
Change-Id: Ic6baee7908a96600ff577217568fec7e5276d48d