tests/functional: Fix the ppc64_hv and the ppc_40p test for read-only assets

Since commit 786bc2255256, cached asset files are read-only, so now we've
got to use "read-only=true" in tests that try to use these files directly.

Fixes: 786bc22552 ("tests/functional: make cached asset files read-only")
Message-ID: <20241105160926.393852-1-thuth@redhat.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
2 files changed