network/iproute: cleanup test, use new library functions

Minor additions:
* tc4: use 'replace' neighbor 127.0.0.1 (it can exist)
* cleanup: remove dummy kernel module on exit
* cleanup: remove added route on exit
* don't rely on ifconfig output, get mtu size from sysfs

Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>
Reviewed-by: Cyril Hrubis <chrubis@suse.cz>