Add tests for the `block-tracker-inspector` middleware — which makes
sure that the block tracker never has a reference to the latest block
which is less than a block number that shows up in an RPC method's
response — and the Infura middleware — which takes care of sending the
request to Infura, and will retry the request up to 5 times if Infura
sends back a certain type of error.
Note that the `retry-on-empty` middleware is not tested because it
currently has a [bug][1] which is making it ineffective.
[1]: https://github.com/MetaMask/eth-json-rpc-middleware/issues/139
Elliot Winkler