Subcultural artifacts are big business these days. They're quickly being hoarded by cultural institutions, rock stars, and corporate lawyers.
It probably won't surprise anyone that authentic old show posters, ratty original t-shirts, hand-made leather jackets, and limited edition records and cassettes go for large sums. If you're over 40, you might be surprised that fliers for shows you went to as a kid can bring in a lot of money. If you were friends with a band or in the band and got a test pressing, and it's for a record that itself is worth a lot? Well then, you can press 1,000 copies of your new band's album just by selling that one copy of the test press.
Test pressings are exactly that, the earliest pressings made off of a record that's just been mastered and plated. Once a test is approved, the order goes through. If a test isn't approved (say, if there is unintentional distortion or a skip — a serious flaw not in the original master recording) then it's mastered and plated again. Tests do not have label art on them and often no other artwork unless someone makes special art up for it.
If you have a mint test pressing of a punk rock record in your attic, you just might be one lucky son of a gun.