autotest: Disable XMPP support in Buffet during tests

BUG=brillo:769
TEST=buffet prints appropriate log messages during buffet_Registration

Change-Id: Ib76e09aeed16c8516d3d1b5dbb0826523af16d6b
Reviewed-on: https://chromium-review.googlesource.com/264394
Tested-by: Christopher Wiley <wiley@chromium.org>
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Christopher Wiley <wiley@chromium.org>
1 file changed