Disable two tests that require IPv6.

bug:25178637
Change-Id: I4615cb4d2f2d9cbb050d02cb77b36215c24fd43b
diff --git a/tools/libcore_failures.txt b/tools/libcore_failures.txt
index b4f686f..81ea79a 100644
--- a/tools/libcore_failures.txt
+++ b/tools/libcore_failures.txt
@@ -164,5 +164,12 @@
   names: ["libcore.java.text.NumberFormatTest#test_currencyFromLocale",
           "libcore.java.text.NumberFormatTest#test_currencyWithPatternDigits"],
   bug: 25136848
+},
+{
+  description: "Lack of IPv6 on some buildbot slaves",
+  result: EXEC_FAILED,
+  names: ["libcore.io.OsTest#test_byteBufferPositions_sendto_recvfrom_af_inet6",
+          "libcore.io.OsTest#test_sendtoSocketAddress_af_inet6"],
+  bug: 25178637
 }
 ]