Polish NetdBinderTest#InitialNetworksExist test

The test meant to make sure that the network with specified netId exist,
and trying to connect to some destination on that network results in
ENETUNREACH. But the test falsely pass when the specified network does
not exist. This patch fix it.

Bug: 179445508
Test: atest NetdBinderTest#InitialNetworksExist
Change-Id: I563e407cccf2eb7991baf9587182a32cecb7725c