releasetools: Log to stdout when running tests.

This sets up the root logger while running tests. It also logs to
stdout, as our Python test result parser doesn't like outputs from
stderr.

Test: `python -m unittest discover build/make/tools/releasetools \
          > /dev/null` doesn't print log lines.
Change-Id: Ic99711bd458bc4b67b38226786fed604c2168476