internal/uevent: enumerate objects via sysfs

This is not a great way to implement cold boot, but I already have the implementation lying around.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/uevent/action_test.go

@@ -29,4 +29,15 @@ func TestKobjectAction(t *testing.T) {
 			t.Errorf("String: %q, want %q", got, want)
 		}
 	})
+
+	adeT(t, "synthetic", uevent.Synthetic, "synthetic",
+		uevent.UnsupportedActionError("synthetic"), nil)
+
+	t.Run("validate synthetic", func(t *testing.T) {
+		t.Parallel()
+
+		if uevent.Synthetic.Valid() {
+			t.Errorf("Valid unexpectedly succeeded")
+		}
+	})
 }