Blockchain stop_gap testing improvements

This is a continuation of the #651 fix. We should also check whether the
same bug affects esplora as noted by @afilini. To achieve this, I've
introduced a `ConfigurableBlockchainTester` trait that can test multiple
blockchain implementations.

* Introduce `ConfigurableBlockchainTester` trait.
* Use the aforementioned trait to also test esplora.
* Change the electrum test to also use the new trait.
* Fix some complaints by clippy in ureq.rs file (why is CI not seeing
  this?).
* Refactor some code.

src/blockchain/esplora/ureq.rs

@@ -88,7 +88,7 @@ impl Blockchain for EsploraBlockchain {
     }
 
     fn broadcast(&self, tx: &Transaction) -> Result<(), Error> {
-        let _txid = self.url_client._broadcast(tx)?;
+        self.url_client._broadcast(tx)?;
         Ok(())
     }