tests/functional/test_ppc64_replay: Mark the e500 test as flaky

tests/functional/test_ppc64_replay: Mark the e500 test as flaky

The test_ppc64_e500 occasionally fails (it just hangs and finally times
out). The same issue could also be reproduced with the former Avocado
test already (especially if the host system is under heavy load), so
it's not a new regression. It's very likely the issue that has been
filed at https://gitlab.com/qemu-project/qemu/-/issues/2523 instead (e.g.
I could not reproduce the issue in older commits before commit 578912ad),
so use this URL for the reasoning in the the decorator.

Message-ID: <20250320065012.309520-1-thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>