Add function to reset tempfile module after fork

On fork, the tempfile module's pseudo random generator is
not reset. If several processes (e.g. two children or parent
and child) try to create a temporary file, they'll conflict.
This function can be used to reset the name generator which
contains the pseudo random generator.

A unittest is included. It is in a separate script because
it changes a variable in the tempfile module to speed up the
test.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>