acpi/tests/avocado/bits: keep the work directory when BITS_DEBUG is set in env

acpi/tests/avocado/bits: keep the work directory when BITS_DEBUG is set in env

Debugging bits issue often involves running the QEMU command line manually
outside of the avocado environment with the generated ISO. Hence, its
inconvenient if the iso gets cleaned up after the test has finished. This change
makes sure that the work directory is kept after the test finishes if the test
is run with BITS_DEBUG=1 in the environment so that the iso is available for use
with the QEMU command line.

CC: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Ani Sinha <ani@anisinha.ca>
Message-Id: <20221117113630.543495-1-ani@anisinha.ca>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>