BpfNetworkStatsTest - cleanup

We note that:
  0 <= BpfMap<K,V>.getMap()
is equivalent to
  BpfMap<K,V>.isValid()

but the latter is far more kosher.

Unfortunately there is more non-kosher stuff happening in this
file, so we still need to define BPF_MAP_MAKE_VISIBLE_FOR_TESTING anyway.

Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I138709c6a298a2d8511b525a8349e01ab87d9455
1 file changed