Add unit test for add_vxlan in test_linux_ip_lib

This patch adds unit tests for the add_vxlan function in the test_linux_ip_lib.py
file.

Fixes bug: 1291974

Change-Id: I0238bedab7a3566aff1115a091ada494a01d2e02
(cherry picked from commit dd52ddb1e9322b3985ceba2db842e8201220f67f)
Merge "Add unit test for add_vxlan in test_linux_ip_lib" into milestone-proposed