A real test pressing is just that. They serve to clean the machines (this is important!) and need to be checked to make sure the stampers aren't damaged so production won't be wasted. They get recycled, except the company usually keeps a 'reference copy'. Those ones are what you often see sold as 'test pressings'. They're from the now defunct or closed or otherwise overloaded company library.
After that's OK there might be a run for artist/company approval. After that, you might get the advance promotion copies done, then the actual production
In the '70s, every major label had a different policy with regard to test pressings. Test pressings were frequently pressed for advanced promotion, i.e. for quick turnaround, since they didn't have to wait for the final art. When a run of test pressings was made for internal promotion, they'd typically have a mimeographed sheet affixed to the cover (or inserted in the cover) that detailed the track listing, producer credits, etc. The labels themselves would be generic "test pressing" labels. Some later test pressings, particularly from PRC, would note the label and sometimes the release number and artist on the label.