Make bitcoin.testnet.sh also works for LND & CLN
See #2508 & #4579 for background.
Based on activating bitcoin.testnet.sh script (on SSH setup) the pre-start script of LND & CLN should quick check if main-bitcoind is running on testnet and shoudl adapt accordently: Running internally also on testnet, while simulating mainnet to the outside (thru adapted configuration).
With testnet4 most prob coming with Bitcoin Core v28 and a lot of additional apps preparing to support it - it makes sense to push this v1.12.0 for now - see for good overview https://github.com/janoside/btc-rpc-explorer/issues/666
It looks like chain lightning is going to have testnet4 support in v25.02 based on this pr: https://github.com/ElementsProject/lightning/pull/7894
Is the focus here going to be only to support testnet4, and not testnet3?
If possible testnet shpould get repaired with the new testnet4 for v1.12.1