simple_regression_test: format and save test results
simple_regression_test: format and save test results

Only collect some significant parameters, format the results
to a brief table for easy comparision.

op-rate  partition-rate  row-rate  latency-mean  latency-median  l-95th-pct  l-99th-pct  l-99.9th-pct
   1836            1836      1836           0.5             0.5         0.6         0.6           2.0
   1836            1836      1836           0.5             0.5         0.6         0.6           2.0
   1836            1836      1836           0.5             0.5         0.6         0.6           2.0

Signed-off-by: Amos Kong <eb01d8f828a6c9a20be4534e72e049aaf41503df@scylladb.com>

amoskong/scylla-cluster-tests,amoskong/scylla-cluster-tests,amoskong/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,amoskong/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,amoskong/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests