In case the proxy was created with wp_proxy_new_wrap(), the event
listener was not attached on the pw_proxy

* move the watchdogs in the fixture setup
* use a new GMainContext on each fixture to avoid test dependencies
* test wp_proxy_sync

* add proxy sync method
* add wrapers for enum/set/subscribe_params
* move the info structure handling to the subclasses
* expose info->props as WpProperties

This also adds a "test server" helper, which is used to
start a local pipewire server in the same process, for testing
functionality that requires a server.

When two clients have the same role, the current policy should
favor the newest client.

Error handling and fixes

See merge request gkiagia/wireplumber!21

Avoids crashing if a rescan is scheduled but the policy module is
destroyed in the meantime