Linux sandbox: add more baseline policy unittests.

sandbox/linux/seccomp-bpf-helpers/sigsys_handlers.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Note: any code in this file MUST be async-signal safe.
 
 #include "sandbox/linux/seccomp-bpf-helpers/sigsys_handlers.h"
 
 #include <unistd.h>
 
@@ skipping 162 matching lines @@
 }
 
 const char* GetPrctlErrorMessageContentForTests() {
   return SECCOMP_MESSAGE_PRCTL_CONTENT;
 }
 
 const char* GetIoctlErrorMessageContentForTests() {
   return SECCOMP_MESSAGE_IOCTL_CONTENT;
 }
 
+const char* GetKillErrorMessageContentForTests() {
+  return SECCOMP_MESSAGE_KILL_CONTENT;
+}
+
 }  // namespace sandbox.