tests/sstables: test counter cell header with large number of shards
tests/sstables: test counter cell header with large number of shards

The logic responsible for reading counters from sstables was getting
confused by large headers. The size of the header depends directly on
the number of shards. This tests checks that we can handle cells with
large number of counter shards properly.

avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla