Al-Mu'minun: (30) Indeed in that...

(Indeed in that) which has been mentioned concerning what was done to him [Noah], peace be upon him, and his people, (are signs) [ayat] that are majestic, which those of insight may use as evidence and from which those capable of reflection may take warning.

(And indeed We were testers [mubtalin].)

Inna (Indeed) is the shortened form of anna (that), and the lam serves as a separator between it and the negative inna—though inna here is not negative, nor is the lam functioning as illa (except). The clause is adverbial (haliyyah), meaning: "And the state of affairs is that We were afflicting the people of Noah with a great trial and severe punishment," or "testing Our servants through these signs, so that We may observe who takes warning and who remembers."

The intended meaning is that We were treating them as one treats those being tested. This is like the saying of the Exalted: "And We have left it as a sign, so is there any who will remember?"