Previously, debug messages from `Plugin.__del__()` went to stdout
which was a problem for the test suite in extremely rare and hard to
predict circumstances.  This corrects that by sending the message to
stderr where all the rest of the messages tend to go.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

plugin.py: Redirect DEBUG from `__del__` to `sys.stderr`

See merge request !944

Sandbox errors (like missing host tools) are dependent on the host
system and rarely on what is actually done.

It is therefore better to not cache them as they are subject to
change between two runs.

Also add test to ensure sandbox failure are not cached