> Personally I think the best solution is to have the test framework allocate a single test directory

This is partially done.  See here:

http://hg.python.org/cpython/file/19c74cadea95/Lib/test/regrtest.py#l1810

# Run the tests in a context manager that temporary changes the CWD to a
# temporary and writable directory.

regrtest sets things up such that the current working directory is supposed to be this temp directory (constructed from _make_temp_dir_for_build()).  But the tests adhere to this only weakly.  Many or most tests create their own temp directory rather than relying on the caller having set the current working directory to a temp directory.  If the tests used a common API, we could control this behavior globally.