tests/functional: automatically clean up scratch files after tests

tests/functional: automatically clean up scratch files after tests

The build/tests/functional subdirectories are consuming huge amounts
of disk space.

Split the location for scratch files into a 'scratch' sub-directory,
separate from log files, and delete it upon completion of each test.
The new env variable QEMU_TEST_KEEP_SCRATCH can be set to preserve
this scratch dir for debugging access if required.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20241121154218.1423005-3-berrange@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20241121165806.476008-3-alex.bennee@linaro.org>