[Test] Expose service account to the testing framework

[m-Peter]

Issue To Be Solved

Currently, when writing integration tests with the Cadence testing framework, there is no way to send some amount of FLOW tokens to any newly-created account. Since FLOW is the native token of the blockchain, it is a must-have feature to be able to send such tokens to any account, and test relevant scenarios that require usage of FLOW tokens.

Suggest A Solution

Since the emulator is used under the hood for integration tests, one possible solution is to expose the service account to the testing framework. The service account contains a large amount of FLOW tokens, and developers will be able to write transactions and send any desired amount to newly-created accounts. The service account is necessary for signing such transactions. Helper methods might be defined as well, to avoid boilerplate code, and create new accounts with a predefined number of FLOW tokens.

Context

onflow/developer-grants#148

[devbugging]

Makes sense to use the service account for that. Would you be able to use this method? https://github.com/onflow/flow-emulator/blob/master/blockchain.go#L182

[m-Peter]

@sideninja Yes, I am using the service key from the Blockchain type, because the account has to be exposed through a custom Test.Account cadence struct. I have already opened a PR that tackles this: #122