tests: perf_fast_forward: Fix result_collector::add() for multi-element results
tests: perf_fast_forward: Fix result_collector::add() for multi-element results

The results vector should be populated vertically, not horizontally.

Responsible for assertion failure with --cache-enabled:

  void result_collector::add(test_result_vector): Assertion `rs.size() == results.size()' failed.

Introduced in 3fc78a25bf7cfc842a90d803874aab72c2ad3e1f.
Message-Id: <164f3dd64f71f0c5b52e5183e93b76fccea9fee1@scylladb.com>

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