Add a small 'passTest' helper

This is symmetric to failTest, and allows us to use it in cases where
we need to return a property.

While replacing 'property True' with it, I saw a case where we can
simplify the use and thus reworked that check.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>