Copy wtperf artifacts when running Jenkins tests

Some of our automated test jobs run multiple wtperf instances per job. Sometimes when investigating a test issue it is interesting to see the raw output from the test run, that output is currently overwritten before the test finishes.

It would be handy to keep a copy of the raw wtperf output and statistics files for all jobs that have been run. We don't want to keep the data files, as that would consume too much disk space.