tests/avocado: test_arm_emcraft_sf2: handle RW requirements for asset

tests/avocado: test_arm_emcraft_sf2: handle RW requirements for asset

The asset used in the mentioned test gets truncated before it's used
in the test.  This means that the file gets modified, and thus the
asset's expected hash doesn't match anymore.  This causes cache misses
and re-downloads every time the test is re-run.

Let's make a copy of the asset so that the one in the cache is
preserved and the cache sees a hit on re-runs.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-ID: <20240726134438.14720-9-crosa@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>