tests/avocado: clean-up socket directory after run

Previously we were leaving temporary directories behind. While the
QEMUMachine does make efforts to clean up after itself the directory
belongs to the calling function. We use TemporaryDirectory to wrap
this although we explicitly clear the reference in tearDown() as it
doesn't get cleaned up otherwise.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20201117173635.29101-4-alex.bennee@linaro.org>