zb/tests: add 'assume_defaults = true' to suppress warning on proxies
This fixes the following warnings on tests:
#[dbus_proxy(...)] macro invocation on 'SecretProxy' without explicit defaults. Please set 'assume_defaults = true', or configure default path/service directly.
#[dbus_proxy(...)] macro invocation on 'SessionProxyBlocking' without explicit defaults. Please set 'assume_defaults = true', or configure default path/service directly.
#[dbus_proxy(...)] macro invocation on 'SessionProxy' without explicit defaults. Please set 'assume_defaults = true', or configure default path/service directly.
#[dbus_proxy(...)] macro invocation on 'IBusProxyBlocking' without explicit defaults. Please set 'assume_defaults = true', or configure default path/service directly.
#[dbus_proxy(...)] macro invocation on 'IBusProxy' without explicit defaults. Please set 'assume_defaults = true', or configure default path/service directly.
#[dbus_proxy(...)] macro invocation on 'MyIfaceProxy' without explicit defaults. Please set 'assume_defaults = true', or configure default path/service directly.
Follow up to !587 (merged).