fix(testnet4): do not compute difficulty adjustment

Raised by: bitcoindevkit/bdk-cli#197

Users of `Testnet4` will run into problems when loading headers from the
database, as the network allows for minimum difficulty blocks. Instead
of adding logic for the header timestamps just for the sake of
`Testnet4`, I am opting to simply ignore the difficutly adjustment. This
boolean does not evaluate to `true` on `Bitcoin`, which is easily
verified in `rust-bitcoin`.

Notably `Signet` does not allow minimum difficulty blocks, so difficulty
adjustment logic is still tested by a testnet as well as mainnet. This
is only dropping `Testnet4`

Author: open-archer702

src/chain/graph.rs

@@ -324,6 +324,12 @@ impl BlockTree {
     }
 
     fn compute_next_work_required(&self, new_height: Height) -> Option<CompactTarget> {
+        // Do not audit the diffulty for `Testnet`. Auditing the difficulty properly for a testnet
+        // will result in convoluted logic. This is a critical code block for mainnet and should be
+        // as readable as possible
+        if self.network.params().allow_min_difficulty_blocks {
+            return None;
+        }
         let adjustment_period =
             Height::from_u64_checked(self.network.params().difficulty_adjustment_interval())?;
         let epoch_start = new_height.checked_sub(adjustment_period)?;