Improve sequence.js test coverage

[corbanbrook]

While doing the migration from ethers v5 to v6 I notice that many of the sequence.js packages have no unit tests.

• abi
• api
• guard
• indexer
• metadata
• migration
• multicall
• network
• replacer

Now i do notice that we do have some integration tests that cover some of these packages working in conjuction with other features as well as sequence-e2e tests but it would be good to add some lower level unit tests as well.