cql-pytest: add a shedding test
cql-pytest: add a shedding test

This scylla-only test case tries to push a too-large request
to Scylla, and then retries with a smaller request, expecting
a success this time.

Refs #8193

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