Add tests for expanded `getblockstats`

[jnewbery]

https://github.com/bitcoinops/bitcoin/tree/expand-getblockstats added new statistics to the getblockstats RPC method, but didn't include functional tests. It'd be good to have tests so we can occasionally rebase the branch on master (eg https://github.com/bitcoinops/bitcoin/pulls) and have confidence that we're not breaking anything.

It should be fairly straightforward to update https://github.com/bitcoinops/bitcoin/blob/8968f345d4303e2a4eabcb3f33a7c9e7ddc35b59/test/functional/rpc_getblockstats.py to test the new statistics.

[jnewbery]

@0xB10C @bitschmidty : let me know if either of you are interested in taking this on.

[0xB10C]

I'll probably pick this up in the next days assuming no one has started working on it yet. Good way for me to learn more about the functional test framework.

[bitschmidty]

@0xB10C Ive started on this today as a way to get familiar with both getblockstats and the functional tests as well. But dont let my starting hold you back!

[jnewbery]

I recommend you get together before @0xB10C replicates any work that @bitschmidty has done already.

[0xB10C]

Tests added in #5.