T6019: Fix smoketest test_system_conntrack custom timeout

After updateing netfilter in the commit https://github.com/vyos/vyos-build/commit/b31f5fe934bcb37534d49acdb5f7756bf05422e8
The nftables format for conntrack timeouts is different.
Fix this.

(cherry picked from commit 24860e092426bf0bb09c2d164d66330be13bcd77)