Always enable logging for unittests

By enabling all log levels, we ensure all calls are fully evaluated.
There was one case in the workerpool where a call to “logging.debug”
was wrong, but not caught in unittests because debug logging was
disabled.

The optional environment variable “LOGTOSTDERR” can be set to
write all log messages to stderr instead of /dev/null.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>