test: ensure sender-pid hint is set in Linux notifications (#43928)

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.

spec/api-notification-dbus-spec.ts

@@ -124,6 +124,7 @@ ifdescribe(!skip)('Notification module (dbus)', () => {
           append: 'true',
           image_data: [3, 3, 12, true, 8, 4, Buffer.from([255, 0, 0, 255, 0, 0, 0, 255, 0, 0, 0, 0, 76, 255, 0, 255, 0, 0, 0, 255, 0, 0, 0, 0, 0, 38, 255, 255, 0, 0, 0, 255, 0, 0, 0, 0])],
           'desktop-entry': appName,
+          'sender-pid': process.pid,
           urgency: 1
         }
       });