EraTransitionGovernance.md: link to existing example test suites

docs/website/contents/for-developers/EraTransitionGovernance.md

@@ -62,6 +62,40 @@ The Conway era uses a dedicated [`Hard Fork Initiation` governance action](https
 
 See [CIP-1694](https://cips.cardano.org/cip/CIP-1694) for the stages of the governance process, specifically the dynamics (eg dedicated quorum threshold) of "Hard Fork Initiations".
 
+## Examples of On-Chain Governance in TestNets
+
+A few teams within IOG already have test suites that involve on-chain governance.
+
+- `cardano-node-tests`.
+  The SDET Team maintains a set of tests that involve a local cluster of nodes building a Cardano chain that has some blocks in every Cardano era.
+  [Their script]( https://github.com/IntersectMBO/cardano-node-tests/blob/05d0c4989d3d1bbebb7af894b4c5f9ba9fd32a25/cardano_node_tests/cluster_scripts/conway/start-cluster) demonstrates how to use `cardano-cli` commands to construct, sign, and submit the necessary transactions for every era except Byron.
+
+  The Byron era there is currently termninated after one epoch via a `TestShelleyHardForkAt: 1` configuration setting, but the Consensus Team is currently working to [remove that functionality](https://github.com/IntersectMBO/ouroboros-consensus/issues/416).
+  Byron on-chain governance as described in this document will replace it, although it will require two epochs, one each to reach protocol versions 1 and 2.
+
+- `cardano-db-sync`.
+  The DB Sync Team creates a "mock" chain to use when testing their tool.
+  [This test](https://github.com/IntersectMBO/cardano-db-sync/blob/0f1d93f9b868caaf14c6cd0e77991e63c07c067f/cardano-chain-gen/test/Test/Cardano/Db/Mock/Unit/Conway/Governance.hs#L451) raises the protocol version within Conway.
+  And [this test](https://github.com/IntersectMBO/cardano-db-sync/blob/0f1d93f9b868caaf14c6cd0e77991e63c07c067f/cardano-chain-gen/test/Test/Cardano/Db/Mock/Unit/Conway/Other.hs#L447) raises the protocol version within Babbage.
+  The linked definitions use more combinators defined elsewhere in that codebase, but it's only a few indirections until the definitions start using the `cardano-ledger` interface directly.
+  Do note that these transactions have no witnesses, which is sufficient for the testing of a tool that trusts its upstream node.
+
+- `ouroboros-consensus`.
+  The Consensus Team wrote tests shortly before the end of the Byron era that tested the mechanism that would transition to Shelley.
+  In particular, [the `mkProtocolByronAndHardForkTxs` function](https://github.com/IntersectMBO/ouroboros-consensus/blob/a9a5f3aaf3ddd45b3dd58a132d65b657bbf285e5/ouroboros-consensus-cardano/src/unstable-byron-testlib/Test/ThreadNet/Infra/Byron/TrackUpdates.hs#L318-L363) and the `mkHardForkProposal` function defined just below it use the `cardano-ledger` interface for Byron in order to create the transactions for the proposal and votes --- each with the necessary witnesses/signatures --- in order to increment the protocol version via Byron on-chain governance.
+  Those functions are used within the Consensus Layer's "ThreadNet" tests, which submit the transactions listed in the `tniCrucialTxs` field at every possible opportunity.
+  That degenerate persistence and eagerness reliably ensures that the chain increments the protocol version as soon as possible.
+
+  A similar ThreadNet test transitions from Shelley to Allegra; its `tniCrucialTxs` field is created by [this function](https://github.com/IntersectMBO/ouroboros-consensus/blob/a9a5f3aaf3ddd45b3dd58a132d65b657bbf285e5/ouroboros-consensus-cardano/src/unstable-shelley-testlib/Test/ThreadNet/Infra/Shelley.hs#L431), which leverages the fact that a single Shelley transaction can demonstrate the necesary quorum for the protocol version increment.
+
+The key features of these three suites with regard to this document are summarized in the following table.
+
+| Repository | Byron | Shelley, ..., and/or Babbage | Conway |
+| - | - | - | - |
+| `cardano-db-sync` | No | Haskell, unsigned | Haskell, unsigned |
+| `ouroboros-consensus` | Haskell, signed | Haskell, signed | No |
+| `cardano-node-tests` | No | `cardano-cli`, signed | `cardano-cli`, signed |
+
 ## Initializing the Chain with a Later Protocol Version
 
 It is often convenient for testnet chains to entirely skip some initial protocol versions.