- test.util.test_io: added testcases for a mkdtemp function

The object, which is returned by mkdtemp, should behave like a str and
a TemporaryDirectory instance (for example, it is removed automatically
if the returned instance is garbage collected). Additionally, the
returned instance can be passed to os.stat.
- util.io: added mkdtemp function (and helpers)

mkdtemp returns a temporary directory.
The returned instance can be treated like a str and a
TemporaryDirectory instance. For example, it can be passed
to os.stat and the temporary directory is automatically
deleted, if the instance is garbage collected.