Ensure referenda TracksInfo is sorted (#3325)

Changes:
- Add `integrity_check` to trait `TracksInfo` to assert the sorting 
- Use the check in `integrity_test`
- Rely upon sorted property in the `info` for speedup

Note that the pallet already relies upon this (so far) untested
assumption
[here](https://github.com/paritytech/polkadot-sdk/blob/44c7a5eb8c375d77f685abf585961421e5f8b228/substrate/frame/referenda/src/lib.rs#L1280).

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Bastian Köcher <git@kchr.de>

prdoc/pr_3308.prdoc

@@ -1,4 +1,4 @@
-title: Parachains-Aura: Only produce once per slot
+title: "Parachains-Aura: Only produce once per slot"
 
 doc:
   - audience: Node Dev

prdoc/pr_3325.prdoc

@@ -0,0 +1,10 @@
+title: "Ensure `TracksInfo` tracks are sorted by ID."
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      Add a `integrity_check` function to trait `TracksInfo` and explicitly state that tracks must
+      always be sorted by ID. The referenda pallet now also uses this check in its `integrity_test`.
+
+crates:
+  - name: pallet-referenda