Back then, Milagro interop used offset 1000 for mock BLS keys.
Meanwhile, interop code was removed and multi client testnets are there.
EF tests use an offset of 1 for mock BLS keys. This patch aligns our
implementation to also use offset of 1, potentially making debugging of
state differences a bit easier (but, ultimately, low impact).
Furthermore, simulation files are now emitted into a subdirectory
to have less chunk in the repo root directory, and to avoid conflicts
where a cached file with offset 1000 runs against tests expecting 1.
See https://github.com/status-im/nimbus-eth2/pull/2928/files#r719266863
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