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## Description
ref: #9183
After some more recent conversations w/ @aaronc, I decided to go back to his original proposal of setting up a subcommand for running in-process testnets.
This PR splits the `simd testnet` command into two subcommands:
- `simd testnet start` which starts an in-process n-node testnet
- `simd testnet init-files` which sets up configuration & genesis files for an n-node testnet to be run as separate processes (one per node, most likely via Docker Compose)
---
Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.
- [x] Targeted PR against correct branch (see [CONTRIBUTING.md](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#pr-targeting))
- [x] Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
- [x] Code follows the [module structure standards](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules/structure.md). - **n/a**
- [ ] Wrote unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing)
- [x] Updated relevant documentation (`docs/`) or specification (`x/<module>/spec/`) - **see #9411**
- [x] Added relevant `godoc` [comments](https://blog.golang.org/godoc-documenting-go-code).
- [x] Added a relevant changelog entry to the `Unreleased` section in `CHANGELOG.md`
- [x] Re-reviewed `Files changed` in the Github PR explorer
- [ ] Review `Codecov Report` in the comment section below once CI passes
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