travis/gitlab/azure: Use --board buildman flag with test.py

The current method of selecting the board to build with test.py is a bit
error-prone, e.g. with "^sandbox$" it actually builds 5 boards (all of
those in the sandbox architecture).

Use the (newish) --board flag instead, to get the same result.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>