row_cache_test: add tests for small_buffer
row_cache_test: add tests for small_buffer

When a buffer of a flat reader is small then the reader can't
handle range_tombstones correctly.

This is not a problem on a production when the buffer is large.

Signed-off-by: Piotr Jastrzebski <2c0eb4a25a7cde333ff2d0860cb06051925fd74f@scylladb.com>

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