test/perf_mutation: Prepare to have several regular columns
test/perf_mutation: Prepare to have several regular columns

Teach the schema builder and test itself to work on more
than one regular column, but for now only use 1, as before.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

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