gh-89792: Prevent test_tools from copying 1000M of "source" in freeze test (#101837)

Prevent test_tools from copying 1000M of "source"
    
It doesn't need a git repo, just the checkout.  We skip .git metadata, Doc/build, Doc/venv, and `__pycache__` subdirs, that developers often have in their clients to reduce the size of the source tree copy ten-fold.

This should significantly reduce IO and presumably time on buildbots during this long test.