Move some NetworkStats tests to tests/net.

This groups them together with the rest of the networking unit
tests. It also speeds up compile/test cycles ("runtest -x" of one
file goes from 1m15s to 30s).

Test: runtest frameworks-net passes on internal tree
Change-Id: I53cb0c51355fe4b4b30e451fa09fbbf58da39efd
7 files changed