Use temporary directories for unit test artifacts

## Feature Request

Unit test binaries are currently compiled in the working directory, and tracked (although not cleaned up) in the unit test builder.  

It would be wiser to simply build the files in the OS's temp area.

This code was an attempt at the problem but conflicted with #294 
https://github.com/Arduino-CI/arduino_ci/commit/1ee342c18740e665b265cc81af346c9ee160c208
