Add support for multiple testnet flags (#1396)

## Issue Addressed

NA

## Proposed Changes

Allows for multiple "hardcoded" testnets.

## Additional Info

This PR is incomplete.

## TODO

- [x] Add flag to CLI, integrate with rest of Lighthouse.


Co-authored-by: Pawan Dhananjay <pawandhananjay@gmail.com>
Co-authored-by: Michael Sproul <michael@sigmaprime.io>

lighthouse/environment/tests/environment_builder.rs

@@ -14,7 +14,7 @@ fn builder() -> EnvironmentBuilder<MainnetEthSpec> {
 }
 
 fn eth2_testnet_config() -> Option<Eth2TestnetConfig<MainnetEthSpec>> {
-    Eth2TestnetConfig::hard_coded().expect("should decode hard_coded params")
+    Eth2TestnetConfig::hard_coded_default().expect("should decode hard_coded params")
 }
 
 mod setup_eth2_config {