Ignore failures of WeakHashMapTest#test_keySet in libcore tests.

The outcome of this test depends on a weak reference being
collected before a timeout, which sometimes fails.  This
test used to pass, but recent changes have made this
non-determinism more frequent.  Until we rewrite this
properly (if ever), just ignore these failures.

Bug: 25437292
Change-Id: I1f041f6dfc1318026bce966575477486207f5351
diff --git a/tools/libcore_failures.txt b/tools/libcore_failures.txt
index 9a8b462..a5476f7 100644
--- a/tools/libcore_failures.txt
+++ b/tools/libcore_failures.txt
@@ -164,5 +164,11 @@
   names: ["libcore.io.OsTest#test_byteBufferPositions_sendto_recvfrom_af_inet6",
           "libcore.io.OsTest#test_sendtoSocketAddress_af_inet6"],
   bug: 25178637
+},
+{
+  description: "Non-deterministic test because of a dependency on weak ref collection.",
+  result: EXEC_FAILED,
+  names: ["org.apache.harmony.tests.java.util.WeakHashMapTest#test_keySet"],
+  bug: 25437292
 }
 ]