tests/functional: add zstd support to uncompress utility

tests/functional: add zstd support to uncompress utility

Rather than using the python library (which has a different API
anyway) lets just call the binary. zstdtools is already in out
qemu.yml so all test containers should have it around. Tests should
still use @skipIfMissingCommands('zstd') to gracefully handle when
only minimal dependencies have been installed.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20250108121054.1126164-20-alex.bennee@linaro.org>