[test_port_list_filter_by_ip_substr] Check if IP free before use it in the test

The test chooses 2 IPs from test subnet but this IP can be used
by portprober or DHCP servises. It can cause test fail.
This patch adds a check if selected IP is free

Related-Prod: PRODX-41551
Change-Id: Ib216d0df9d1c2716cccb3a98ccae625f06308d1f
1 file changed