Add utility functions to help unit tests verify netstack Stats

**Description**

Many of the netstack unit tests need to check that the stats got updated correctly after some functions is called. Having a standard way to do this will reduce the code size of many of our unit tests and keep the code readable while improving value we get out of the tests.

**Do you have a specific solution in mind?**

One way to do this would be to have a way to capture a snapshot of the stats (or a subset, say `tcpip.Stats.IP`) at a given moment in time (eg. right before we call the function(s) under test) and another function that outputs the diff, in a format that is easy for test cases to encode in their parameters.