Ensure config preset schedule fork does not affect testnets

**Describe the bug**

- From https://github.com/ChainSafe/lodestar/pull/4774

Noticed that mainnet forks are scheduled in the mainnet config preset. All testnets extend that preset overwriting some properties. It is possible that a fork is scheduled for mainnet and not for some testnet, inadvertently causing that testnet to fork at a random epoch.

**Expected behavior**

Ensure that scheduled mainnet forks only affect the mainnet network