test_distutils will be difficult; the leak is around the "import xx" in
Lib/distutils/tests/test_build_ext.py.

And Python/import.c says:
/* To prevent initializing an extension module more than once, we keep a
static dictionary 'extensions' keyed [...] by filename (for dynamically
loaded modules). A copy of the module's dictionary is stored [...] 
*/

This dictionary keeps growing with random filenames in the temp
directory. I can't see a way to clean it.