Clear log files as part of clean

[CharliePoole]

We run tests using both 3.15.5 and 3.17.0, which still have the infamous "zero-length log file" bug - nunit/nunit-console#1178. This is quite annoying as they build up run after run. Easy fix is to erase them in the Clean target.

[CharliePoole]

Cleaning the logs file as part of the Clean task is useful because it eliminates all logs from earlier runs. However, running the unit and package tests is still producing about 30 log files, mostly empty. I need to selectively delete zero-length files after those test runs, so I only see the files where I've asked for an actual trace.