tempfiles would be cleaned up between tests, so the new "base directory"
would be removed as well, that made the next tentative to create a temporary
directory fail.

This is not interesting for regular execution of diffoscope, as only
programs executing diffoscope.main.main() twice within the same python
process would notice.

Closes: #902709
Signed-off-by: Mattia Rizzolo <mattia@debian.org>