Add IPv6TetheringCoordinator unit test

Bug: 148636687
Test: atest IPv6TetheringCoordinatorTest

Change-Id: I1ba30bb524cbc0cc2782ce1702f9889c0d8a7576
Merged-In: I939323ce09adb9c66b1e2b83d58b0f892aa8f011
(cherry picked from commit 368c56dd7133d292c5f3ab5942016bb4dd1fbc88)
1 file changed