After the prior commit subtests advanced the chain, the txpool's
internal Reset() is asynchronous. On slower runners (observed on the
Windows amd64 CI image) GetPoolNonce returns a stale value before
Reset has processed the new head, the signed tx gets rejected as
nonce-too-low, and the committed block lands empty.
The from-mempool path is optional per spec ("MAY build from mempool")
and is already covered by the hive commit-block-z-from-mempool SpecOnly
fixture, so removing this unit-test variant doesn't lose coverage.
testing_commitBlockV1 is the write companion of testing_buildBlockV1: it
builds a block from the provided payloadAttributes and txs, inserts it,
and sets it as the canonical head, returning the new head hash. Skipping
the engine_newPayload + engine_forkchoiceUpdated serialize/deserialize
round-trip makes it useful for state-shape benchmarking and reproducible
test-chain construction where the caller wants the chain to advance.
The new miner.CommitTestingBlock shares its generation path with
BuildTestingPayload via an unexported helper, so both code paths produce
the same block from the same inputs.
Spec and cross-client fixtures: ethereum/execution-apis#801
