test: add parallelized select count test
test: add parallelized select count test

Added test that checks if a SELECT COUNT(*) query was transformed and
processed in a parallel way. Checking is done by looking at the cql
statistics and comparing subsequent counts of parallelized aggregation
SELECT query executions.

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