The PRNG is not the only source of the randomness in the tests. The exact behavior depends on the order of files in directories, on string hashes randomization, on address randomization, and on many other things out of our control. Couldn't reseeding the PRNG just add a false promise? The success in making tests deterministic can also narrow down the coverage of the testing. Some branches that lead to failures can be never executed. Our target not just making tests always success, but catch and fix even pretty rare errors.