Add tests for Rosetta Construction API

[Ullaakut]

Goal of this PR

Part of #333

• Adds tests to the new Transactor component, introduced by the Rosetta Construction API (#387) and the new method on the rosetta retriever, Sequence.
• Refactors the ParseTransaction method from the Transactor into its own dedicated type for readability and testability.

Additional Notes

// FIXME: The naming of those funcs conflict with the transactor's, since they have the same dependencies :(
//        How can we name this in a consistent way that makes sense?
func ParseTransaction(tx *sdk.Transaction) func(*TransactionParser) {
    return func(parser *TransactionParser) {
        parser.tx = tx
    }
}

func ParseValidator(validate Validator) func(*TransactionParser) {
    return func(parser *TransactionParser) {
        parser.validate = validate
    }
}

func ParseGenerator(generate Generator) func(*TransactionParser) {
    return func(parser *TransactionParser) {
        parser.generate = generate
    }
}

func ParseInvoker(invoke Invoker) func(*TransactionParser) {
    return func(parser *TransactionParser) {
        parser.invoke = invoke
    }
}

Any suggestions? Since we already have WithX for injecting those dependencies to the Transactor in the same package.

Checklist

• [x] Is on the right branch
• [x] Documentation is up-to-date
• [x] Tests are up-to-date

[awfm9]

Any suggestions? Since we already have WithX for injecting those dependencies to the Transactor in the same package.

If these are purely for testing, you can just use InjectXXX etc?