Support for Python 3.4 unittest subtests

[PiDelport]

Unittest in Python 3.4 added support for subtests, a lightweight mechanism for recording parameterised test results.

At the moment, pytest does not support this functionality: when a test that uses subTest() is run with pytest, it simply stops at and reports the first failure.

Pytest already supports its own, pytest-specific parametrizing mechanism, but it would be convenient if the pytest runner also supports the standard library's subTest() functionality.

[PiDelport]

There's some relevant but old discussion on the testing-in-python mailing list: "unittest subtests" (January 2013)

[RonnyPfannschmidt]

It's important to note that pytest parametrize and subtests are fundamentally completely different

[The-Compiler]

While this is a nice feature, it shouldn't block 3.0. I also think this would be some more serious work due to pytest's separation of collection/running, so I'm removing the milestone here.