test: Return values from the asserts compatible with cmds

test: Return values from the asserts compatible with cmds

The asserts are sometimes called from the context of the test command
itself so make sure that a return that happens as a result of a failure
is compatible with that command return. When called within a test, the
return value is ignored.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Simon Glass <sjg@chromium.org>