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).

Bug: 33681750
Test: runtest frameworks-net passes on internal tree
Merged-In: I53cb0c51355fe4b4b30e451fa09fbbf58da39efd
Change-Id: I5ae5dbf74c94feb0fe3759681e5e6d6fba62fa32

(cherry picked from commit c86013be)