This test shows how the evm t8n can be used to calculate the (ethash) difficulty, if none is provided by the caller,
this time on GrayGlacier (Eip 5133).
Calculating it (with an empty set of txs) using GrayGlacier rules (and no provided unclehash for the parent block):
[user@work evm]$ ./evm t8n --input.alloc=./testdata/19/alloc.json --input.txs=./testdata/19/txs.json --input.env=./testdata/19/env.json --output.result=stdout --state.fork=GrayGlacier