9ea564ff

By: Tom Sydney Kerckhove <syd@cs-syd.eu>

Generate the commands in the download tests, and hardcode the batch size

The download tests were built on a hand-written Command repeated with a
counter. They now take generated commands, with only the length of the list
constrained so that it always spans more than one batch.

The batch size is no longer a setting. It sits in Env at 1024, the way
envHashDifficulty is hardcoded at 10, so the tests can still run against a
small value without exposing a knob nobody needs.

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