Add unit tests for handling non-canonical blocks and refactor code

[prathamesh0]

Related to cerc-io/ipld-eth-server#170

• Add tests for:
  • header
  • transactions
  • receipts
  • logs
  • state and storage nodes
  • tests for non-canonical block at a different height
• Refactor indexer tests to remove duplicate code
  • Refactor pre-existing indexer tests
    • Refactor mainnet tests
    • Refactor legacy block test
    • Refactor watched-addresses tests
  • Refactor tests for non-canonical blocks