Code review comment for ~pieq/checkbox-support:fwts-test-output

The default behavior of calling fwts with `-r ` argument is to create a plaintext file if log type is not assigned by `--log-type` argument. This leads to making fwts always generate a `.log` file no matter what file name is assigned following `-r` argument[1]
This can be seen in my test result: https://pastebin.canonical.com/p/zhpdCzS7Dm/

Therefore with a log name without any suffix or a random user-identified suffix will make the generated log file name asynchronous between fwts_test.py and fwts, thus the following result can be found:
https://pastebin.canonical.com/p/3BvnBF82Rt/
(I got this result with the original suspend/suspend_advanced_auto job in the stable version of p-p-c)

[1] https://github.com/ColinIanKing/fwts/blob/master/src/lib/src/fwts_log.c#L333
When the log type is plaintext, fwts truncates the original suffix of assigned filename if it's recognized by fwts (such as .log, .json, .xml and .html) and append .log as the suffix of the generated log file. When the original suffix is not recognized, append .log directly as the suffix of the generated log file.