Rules such as "check" on our Makefile make sure that no temporary
directory created by Avocado is left behind, but, it's very hard to
tell who created a rogue directory.

This brings improvements to the naming of the temporary directory, so
that it's trivial to tell which test created in the first place.

Additionally, it improves the temporary directory creation, usually by
making use of a tearDown() test phase, instead of other more fragile
methods.
Signed-off-by: NCleber Rosa <crosa@redhat.com>