Add TestNetworkTracker to test utils.

The tracker allows easily creating valid Network objects that are based
on test (TUN) interfaces.
It is taken from GTS NetworkStack test code and adapted to Kotlin.

Test: tests based on this class (e.g. CaptivePortalLoginTests)
Bug: 131401028
Change-Id: I6f8d1e36670c46ebcbca01e0a64ce579bcdcc1f9