Add tests for legacy transaction signing

[piersy]

This PR - #165 added support for singing legacy transactions but did not provide any tests of the functionality. The functionality should be covered to a good degree by the test that compares rpc outputs between celo and cel2 implementations (because it will verify that the from field is correctly generated for all transactions occurring before the transition to cel2). However it would be good to have some unit tests of this functionality to prevent regressions.

This ticket covers adding those tests. We could import celo-blockchain and compare it's output with the new signing implementation, or we could take historical transactions and ensure that the signer produces correct result, this would be for all the transaction types.

Additionally some lower level tests would be useful, such as a test ensuring the ordering of forks is correct. For example checking that transactions deprecated in the cel2 fork are not signed when the cel2 fork is enabled.