Don't panic when shutting down with controller and writing report

write report file in the same place that we display final metrics
don't store open file for duration of load test -- instead, test creation at test-start, then re-open to write report at end of test
closes https://github.com/tag1consulting/goose/issues/474
adds test coverage, writing a report file when using the controller
validate report file is properly created and not-empty
cleanup report file at end of successful test

As part of this change, the last thing Goose reports now is where the report file has been written, immediately following the ASCII metrics:

 === OVERVIEW ===
 ------------------------------------------------------------------------------
 Action       Started               Stopped             Elapsed    Users
 ------------------------------------------------------------------------------
 Increasing:  2022-05-24 09:02:17 - 2022-05-24 09:02:19 (00:00:02, 0 -> 1)
 Increasing:  2022-05-24 09:02:19 - 2022-05-24 09:02:24 (00:00:05, 1 -> 5)
 Maintaining: 2022-05-24 09:02:24 - 2022-05-24 09:02:34 (00:00:10, 5)
 Increasing:  2022-05-24 09:02:34 - 2022-05-24 09:02:35 (00:00:01, 5 -> 20)
 Maintaining: 2022-05-24 09:02:35 - 2022-05-24 09:02:39 (00:00:04, 20)
 Decreasing:  2022-05-24 09:02:39 - 2022-05-24 09:02:45 (00:00:06, 1 <- 20)
 Increasing:  2022-05-24 09:02:45 - 2022-05-24 09:02:50 (00:00:05, 1 -> 20)
 Decreasing:  2022-05-24 09:02:50 - 2022-05-24 09:02:52 (00:00:02, 7 <- 20)
 Decreasing:  2022-05-24 09:02:52 - 2022-05-24 09:02:53 (00:00:01, 4 <- 7)
 Increasing:  2022-05-24 09:02:53 - 2022-05-24 09:02:59 (00:00:06, 4 -> 25)
 Decreasing:  2022-05-24 09:02:59 - 2022-05-24 09:03:36 (00:00:37, 0 <- 25)

 Target host: http://apache.fosciana/
 goose v0.16.3-dev
 ------------------------------------------------------------------------------
07:03:36 [INFO] html report file written to: report.html

tag1consulting / goose

Don't panic when shutting down with controller and writing report #489