k/s/tests: benchmark the subscription-metadata hash

The cases sweep the partition count of one topic and the topic count of
one group, so that a later change to the hash has a baseline. A
hash_topic figure covers the hash and the free of its input, and the
free_topic case at the same partition count measures that free alone.

Signed-off-by: Oren Leiman <oren.leiman@redpanda.com>