Test fix: Handle missing dummy0 network interface

The test should pass/skip if the dummy0 network interface is
not present. It's not present on Fugu and the test was failing
because NetworkInterface.getByName("dummy0") returns null, as
per docs.

This change modifies the test_isReachable_neverThrows test to
skip the reachability check when the dummy0 interface cannot
be found. An error looking up dummy0 will now be reported
as a failure instead of treating it as an automatic pass.

This has been tested on an angler (with dummy0) and a fugu
(without).

Test: Ran libcore/java/net/InetAddressTest.java
Bug: 34683975
Change-Id: I5cedeb7354ac066b1358bd6071dbb013a6f5fa3c