buffet: Add flag to disable XMPP support

As a temporary work around to major source of flake in tests, allow
disabling XMPP support in tests.

BUG=brillo:769
TEST=FEATURES="test" USE="clang asan" emerge-gizmo buffet && \
     FEATURES="test" emerge-gizmo buffet

Change-Id: I41ca92bb77b77132c1a20c7018b0ce737b69cf60
Reviewed-on: https://chromium-review.googlesource.com/264392
Tested-by: Christopher Wiley <wiley@chromium.org>
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Christopher Wiley <wiley@chromium.org>
diff --git a/buffet/device_registration_info_unittest.cc b/buffet/device_registration_info_unittest.cc
index dfb61cc..91dc8b1 100644
--- a/buffet/device_registration_info_unittest.cc
+++ b/buffet/device_registration_info_unittest.cc
@@ -211,6 +211,7 @@
         new DeviceRegistrationInfo(command_manager_, state_manager_,
                                    std::move(config),
                                    transport_, storage_,
+                                   true,
                                    mock_callback));
   }