librbd/cache/pwl: initialize generate_test_instances() objects

librbd/cache/pwl: initialize generate_test_instances() objects

... to prevent check-generated.sh failures such as:

**** librbd::cache::pwl::WriteLogPoolRoot test 1 dump_json check failed ****
   ceph-dencoder type librbd::cache::pwl::WriteLogPoolRoot select_test 1 dump_json > /tmp/typ-cAoWrqlHC
   ceph-dencoder type librbd::cache::pwl::WriteLogPoolRoot select_test 1 encode decode dump_json > /tmp/typ-ES5yHpfGL
5c5
<     "flushed_sync_gen": 0,
---
>     "flushed_sync_gen": 255,

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>