Replace custom algorithm in constants unittest

There is no need for the “_IsUniqueSequence” function anymore, it can
easily be replaced by utils.FindDuplicates. Also, pass the message as a
keyword parameter and use the more commonly used assert* functions.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>