Learn Hardcore Norwegian: Trykktesten virket unødvendig, men var tvert imot viktig. - The pressure test seemed unnecessary but was, on the contrary, important.

Norwegian often uses the definite form when talking about a specific thing already known from context. The ending -en is the usual definite ending for many masculine nouns.

So:

trykktest = base form
trykktesten = the pressure test

What kind of word is trykktesten? Is it a compound?

Yes. Trykktesten is built from the compound noun trykktest:

trykk = pressure
test = test

Together, trykktest means pressure test. Then the definite ending -en is added:

trykktest → trykktesten

Compound nouns are very common in Norwegian, just as in English, but Norwegian usually writes them as one word.

Why does the sentence use both virket and var?