libweave: Re-factor dbus_command_proxy_unittest.cc to public libweave

Now dbus references could be moved out from libweave.

BUG=brillo:1245
TEST='FEATURES=test emerge-gizmo buffet'

Change-Id: I77b862895bc9a742586b3c24cbc4a223cffbbb11
Reviewed-on: https://chromium-review.googlesource.com/288805
Tested-by: Vitaly Buka <vitalybuka@chromium.org>
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>