Merge #11789: [tests] [travis-ci] Combine logs on failure

Merge #11789: [tests] [travis-ci] Combine logs on failure

ff8a9b0 [tests] Add combinedlogslen argument to test_runner.py (John Newbery)
dba94ea [tests] [travis-ci] Move Travis functional test log post processing to test_runner (John Newbery)
bba1c54 [tests] Improve logging shutdown and add hint for combine_logs (John Newbery)

Pull request description:

  Replaces #11779 . Notes from that PR:

  > Currently, when a functional test fails, the debug logs are printed sequentially to the travis log. This makes debugging race conditions based on the travis log hard. Instead, all logs events should be combined and sorted by their timestamp, then appended to the travis log.

  @MarcoFalke

Tree-SHA512: 56c80067d6a2c92f7e6a35e3ae5160637a0de052d9da593c7be6e02233544a93c66d62456f903f85e2edc09e31ab4bdafd1aed1d9897ae48c634f82631f856f7