Fix incorrect fallback path for iptables.

When running the tests on device, we look for iptables in /sbin
(for the net_test system image) and then in /system/bin (for when
the tests are run on device). The /system/bin path lacks a
trailing backslash, which breaks setup when running the tests on
device.

Bug: 36510518
Test: device tests run to completion instead of failing in setUpClass.
Test: all_tests.sh passes on android-4.9
Change-Id: I45200c66ae8aa936ae22dd417c53b3c038cd2562