`Blockchain` stop_gap testing improvements

`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.