Run pythoneval tests using --incremental

There is a simple idea how to speed-up pythoneval.py tests, that are currently very slow. After running the first test we can write cache and use it for the next test, so that we don't reprocess builtins, typing, etc.

I believe the tests are run without the --no-incremental flag, so they are run in incremental mode (since it is on by default now), but discard the cache on cleanup, no? So really we want to keep the created cache?