tests: run tests in parallel
tests: run tests in parallel

Launch tests in a concurrent executor with worker count determined
by available memory.

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