tests/functional: Convert the ppc_hv avocado test into a standalone test

tests/functional: Convert the ppc_hv avocado test into a standalone test

A straight forward conversion, we just also have to remove the decorator
@skipUnless(os.getenv('SPEED')) since all non-trivial functional tests
are running in SPEED=thorough mode now. Also make sure that the extracted
assets are writable, so that the test does not fail if it gets re-run
and there are stale read-only files already around.

Message-ID: <20240830133841.142644-28-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>