tests/functional: make cached asset files read-only

tests/functional: make cached asset files read-only

This ensures that if a functional test runs QEMU with a writable
disk pointing to a cached asset, an error will be reported, rather
than silently modifying the cache file.

As an example, tweaking test_sbsaref.py to set snapshot=off,
results in a clear error:

  Command: ./build/qemu-system-aarch64 ...snip... -drive file=/var/home/berrange/.cache/qemu/download/44cdbae275ef1bb6dab1d5fbb59473d4f741e1c8ea8a80fd9e906b531d6ad461,format=raw,snapshot=off -cpu max,pauth=off
  Output: qemu-system-aarch64: Could not open '/var/home/berrange/.cache/qemu/download/44cdbae275ef1bb6dab1d5fbb59473d4f741e1c8ea8a80fd9e906b531d6ad461': Permission denied

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20241025092659.2312118-3-berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>