feat: migrate testnet code to xeon-testnet repo (#11)

* feat: migrate testnet code to `xeon-testnet` repo

- init new repo at `xeon-protocol/xeon-testnet`
  - add as submodule of `v1-core`
- remove all testnet-specific code

* chore: remove testnet ABI files

.gitmodules

@@ -13,3 +13,6 @@
 [submodule "lib/forge-std"]
 	path = lib/forge-std
 	url = https://github.com/foundry-rs/forge-std
+[submodule "lib/xeon-testnet"]
+	path = lib/xeon-testnet
+	url = https://github.com/xeon-protocol/xeon-testnet