Disable libcore.libcore.io.FdsanTest#testSocket on target.

The test is now timing out on heap-poisoning target builds. Disable it
to avoid the bot spam while we investigate the issue.

Test: Rely on the ART Buildbot's heap-poisoning target builders.
Bug: 116446372
Change-Id: I412b1c32a3477501a131b4e07045ee8ccbc61fba
diff --git a/tools/libcore_failures.txt b/tools/libcore_failures.txt
index 4c9fd96..a5fa332 100644
--- a/tools/libcore_failures.txt
+++ b/tools/libcore_failures.txt
@@ -226,5 +226,12 @@
           "libcore.libcore.io.FdsanTest#testParcelFileDescriptor",
           "libcore.libcore.io.FdsanTest#testDatagramSocket",
           "libcore.libcore.io.FdsanTest#testSocket"]
+},
+{
+  description: "Timeout on heap-poisoning target builds",
+  result: EXEC_FAILED,
+  modes: [device],
+  bug: 116446372,
+  names: ["libcore.libcore.io.FdsanTest#testSocket"]
 }
 ]