When factory software was still using autotest, we find that autotest has a very long latency to startup, so a prespawner was introduced to speed up. Pytests, based on unittest, starts much faster so there's no need to use prespawner. However in order to unify the code execution path, we also made a prespawner for pytests.

Now, autotest support is removed and we should evaluate again if using prespawner is really needed. If not, we may simply load and run python module files, to prevent the overhead of serializing session data, communicate via pipe, and reloading modules.

However, this implies the forked process may have heavy initial memory footprint (which comes from Goofy) and we need to measure how this impacts test startup time. (In theory Linux memory will be managed as copy-on-write, but we're not sure how python would make changes to this)