self test fails for large values of $$

/t/thml/form.t assumes that pids are <= 99999999.

For larger values of $$ the result of:

my $filename = sprintf "foo-%08d.txt", $$;

contains 9 or more digits and as a result the Content-Length
of the test message will not be 139.

Replacing the format string with "foo-%08X.txt" solves the problem.