tempfiles: empty the list of known tempfiles after removing them

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>