Define which EIPs may add tests

In preparation for Hegota, only EIPs in the CFI/SFI stage may add tests. Specs for other EIPs remain welcome under `specs/_features/` but they must not add tests. Tests for new EIPs can be difficult to review as they generally touch many files and sometimes require format changes, which is more invasive than a self-contained specs directory.

Inclusion stages are defined in [EIP-7723](https://eips.ethereum.org/EIPS/eip-7723):

| Stage | Meaning |
| --- | --- |
| **SFI** (Scheduled For Inclusion) | Strong intent to include |
| **CFI** (Considered For Inclusion) | Client teams intend to prototype/test |
| **PFI** (Proposed For Inclusion) | Proposed; no client commitment |
| **DFI** (Declined For Inclusion) | Will not be included |

### Tasks

- [ ] Document this somewhere, so new contributors do not waste time.
- [ ] https://github.com/ethereum/consensus-specs/pull/5621
- [ ] Remove tests for PFI features already on master.
- [ ] [EIP-8025](https://github.com/ethereum/consensus-specs/tree/master/specs/_features/eip8025) (Optional Execution Proofs)
- [ ] https://github.com/ethereum/consensus-specs/pull/5622
- [ ] [EIP-8148](https://github.com/ethereum/consensus-specs/tree/master/specs/_features/eip8148) (Custom sweep threshold)
- [ ] https://github.com/ethereum/consensus-specs/pull/5623
- [ ] [EIP-8205](https://github.com/ethereum/consensus-specs/tree/master/specs/_features/eip8205) (Withdrawal credentials preregistration)
- [ ] https://github.com/ethereum/consensus-specs/pull/5624
- [ ] [EIP-8321](https://github.com/ethereum/consensus-specs/tree/master/specs/_features/eip8321) (Hash-Chain RANDAO)
- [ ] https://github.com/ethereum/consensus-specs/pull/5625
- [ ] Remove tests from open PRs that are not CFI/SFI.
- [ ] https://github.com/ethereum/consensus-specs/pull/5452
- [ ] https://github.com/ethereum/consensus-specs/pull/5592
- [ ] https://github.com/ethereum/consensus-specs/pull/5464