test: expect a `sender-pid` hint in Linux notifications.

This PR ensures that the `sender-pid` hint is set for new notifications.
It also updates the spec to confirm that DBus receives the hint and that
it has the correct value.

This fixes a spec failure when running libnotify >= 0.7.12 (2022-05-05).
Starting with that version, libnotify started injecting `sender-pid` if
not provided by the client. So our tests received a slightly different
DBus payload depending on what version of libnotify was installed,
causing our deep-equals tests to fail.

By always providing and testing the `sender-pid` hint, our behavior and
tests should be consistent across distros.