smoketest: T4387: Add test for load-balancing wan

Create 2 network namespaces which allow us to emulate 2 ISP
with different static addresses
Check routing table 201 for the first ISP and table 202 for the
second ISP. Each table must contain default route

(cherry picked from commit 6b75cbb0575ca95806e969f5d7f219c0cbeea334)