Add tests to make sure UnixDomainSocket and namespaces play nicely

sandbox/linux/sandbox_linux_test_sources.gypi

 # Copyright (c) 2012 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.
 
 # Tests need to be compiled in the same link unit, so we have to list them
 # in a separate .gypi file.
 {
   'dependencies': [
     'sandbox',
     'sandbox_linux_test_utils',
     '../base/base.gyp:base',
     '../base/base.gyp:test_support_base',
     '../testing/gtest.gyp:gtest',
   ],
   'include_dirs': [
     '../..',
   ],
   'sources': [
     'tests/main.cc',
     'tests/unit_tests_unittest.cc',
     'services/broker_process_unittest.cc',
     'services/scoped_process_unittest.cc',
     'services/thread_helpers_unittests.cc',
+    'services/unix_domain_socket_unittest.cc',
     'services/yama_unittests.cc',
   ],
   'conditions': [
     [ 'compile_suid_client==1', {
       'sources': [
         'suid/client/setuid_sandbox_client_unittest.cc',
       ],
     }],
     [ 'use_seccomp_bpf==1', {
       'sources': [
         'seccomp-bpf/bpf_tests.h',
         'seccomp-bpf/codegen_unittest.cc',
         'seccomp-bpf/errorcode_unittest.cc',
         'seccomp-bpf/sandbox_bpf_unittest.cc',
         'seccomp-bpf/syscall_iterator_unittest.cc',
         'seccomp-bpf/syscall_unittest.cc',
       ],
     }],
     [ 'compile_credentials==1', {
       'sources': [
         'services/credentials_unittest.cc',
       ],
     }],
   ],
 }