Add a log/test.log that mirrors the console output

This is extremely handy when debugging riak_test issues, since the
actual test logs can omit messages between tests and also be hard to
reassemble.
Post the test output log separately from the test result, fixes bug when non-UTF8 chars are in logs.
Merge pull request #697 from basho/feature/backport-test-log-upload

Backport test output upload