Add a test for GetBlockReceipt api method

[piersy]

We don't have a test for this, and it's going to be tricky to make one because that method exists to serve block receipts that are an artefact of the old celo-blockchain codebase. As such we don't have any code in this codebase that produces block receipts.

So maybe the best approach would be to use a small datadir generated from legacy celo, however that would probably still be a few MBs which is still kind of too large to be reasonable.

[palango]

Can't we just serialize one block that has a block receipt and load that for the test?