Lib/test/regrtest.py:
# When tests are run from the Python build directory, it is best practice
# to keep the test files in a subfolder.  This eases the cleanup of leftover
# files using the "make distclean" command.
if sysconfig.is_python_build():
    TEMPDIR = os.path.join(sysconfig.get_config_var('srcdir'), 'build')
else:
    TEMPDIR = tempfile.gettempdir()