Avoid requiring NetworkMagic for tests

At the moment, each `blk` requires a `NetworkMagic`. See https://github.com/input-output-hk/ouroboros-network/blob/e31c1de2df13918516c490f98118ed4ae0028bff/ouroboros-consensus/src/Ouroboros/Consensus/Config/SupportsNode.hs#L14

This `NetworkMagic` is only needed by the network layer and to avoid mixing up database folders. Test blocks and the ThreadNet tests don't care about `NetworkMagic` at all. So it would be nice if we could drop this requirement. We can possibly do the same for `getSystemStart`.

Start with Ouroboros/Consensus/Config/SupportsNode.hs at the linked NetworkMagic requirement, then inspect how test blocks and ThreadNet tests construct blocks. Determine whether getSystemStart has the same dependency. Done means those tests no longer require NetworkMagic, with the relevant test suite still passing.

Written by the indexing model from the issue text.