buildman: Adjust tests for new boards.cfg format

Commit 27af930e changed the boards.cfg format
but missed to change the parsing in buildman. A follow-on commit
03c1bb24 fixed this but missed fixing the
tests.

This patch updates the tests to fit the new Board constructor.

./tools/buildman/buildman -t
<unittest.result.TestResult run=1 errors=0 failures=0>

Signed-off-by: Simon Glass <sjg@chromium.org>