[test] Mocknet Litecoin 0.21.2 and Smoke Tuning

There are some problems with the latest version of the litecoin image in regtest mode, possibly due to mimble wimble changes. We are unable to create blocks beyond an incosistent block height around 431-436 where the block weight increases and the error bad-txns-vin-empty starts occurring. I don't want to dump much time into debugging the root issue, but this change makes smoke tests a bit faster and slows down the litecoin blocks enough that we can finish before that height (but not so slow that it exposes other races in the smoke tests). The litecoin daemon will stop working in longer running mocknets, but with this we can at least run smoke tests against the current mainnet version to catch issues like !2806 (merged).