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

[jeremyandrews]

• 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 Panic when shutting down via controller #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

[slashrsm]

Reviewed and tested. I can confirm that the process exits without a panic when generating the report file.