tests: Fix promoted_index_read to not rely on a specific index length
tests: Fix promoted_index_read to not rely on a specific index length

Signed-off-by: Vladimir Krivopalov <dc9186a06078733915a6fcbab34e59120be2b484@scylladb.com>

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