Adding a notification may fail, instead of waiting for the timeout
we can check the result we get in the callback.

For debugging it's important that the backend doesn't just crash without
telling the reason. It would be even nicer if the test would fail
immediately instead of hitting the timeout, but that' currently not
possible since the client doesn't know whether the backend failed.

We can reuse most of the test setup for all tests. This will make it
easier to test more properties.

This filters unsupported properties from a notification request without
failing it. This allows backwards compatibility after adding new
properties. This should had been the case from the beginning so that we
don't have to break API when extending the notification interface. See
[1].

[1] https://dbus.freedesktop.org/doc/dbus-api-design.html#extensibility

This allows setting a custom path for the icon validator, especially
useful when testing without installing.

The sandbox for the icon-validator needs a privileged container.
And it needs librsvg to be able to validate SVGs.

If debug messages are turned on the output of validate-icon isn't a
valid key file since it contains log output as well.