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/testutils/mod.rs

@@ -14,6 +14,10 @@
 #[cfg(feature = "test-blockchains")]
 pub mod blockchain_tests;
 
+#[cfg(test)]
+#[cfg(feature = "test-blockchains")]
+pub mod configurable_blockchain_tests;
+
 use bitcoin::{Address, Txid};
 
 #[derive(Clone, Debug)]