Provide basic builtin fixtures

Currently ward contains no built-in fixtures. There’s a few ones that would be nice, if were provided, like pytest tmp_path:

tmp_path
    Return a temporary directory path object which is unique to each test
    function invocation, created as a sub directory of the base temporary
    directory.

    By default, a new base temporary directory is created each test session,
    and old bases are removed after 3 sessions, to aid in debugging. If
    ``--basetemp`` is used then it is cleared each session. See :ref:`base
    temporary directory`.

    The returned object is a :class:`pathlib.Path` object.

[darrenburns]

Mentioned elsewhere that an equivalent of capsys would also be useful.

[AndydeCleyre]

I just want to note that for the case of tmp_path, I happily include plumbum in my testing dependencies, giving me access to these fine context managers:

from plumbum import local

with local.tempdir() as tmp:
    with local.cwd(tmp):

I always prefer their path objects anyway.