buildman: Perform tests in a temporary directory

We may not always be able to write to the default output directory so
have a temporary directory for our output be created.

Cc: Simon Glass <sjg@chromium.org>
Cc: Stephen Warren <swarren@wwwdotorg.org>
Suggested-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Tom Rini <trini@konsulko.com>