Apparently the term "shahadat" does not mean the testimonies that the religion are based on them like testifying to the lord with monotheism, to the prophet with the prophecy and to the imams with imamate since it is enough to fulfill these testimonies by believing in them. The verse is praising the devotees because of their fulfillment to the obligatory testimony which is testifying against or with others to establish justice. It is one of the good characters of devotees that the verse is praising them because of it.
