[test] Always require signer argument when signing a transaction with the se…

[m-Peter]

Closes #263

Description

Until now, to execute a transaction with the service account, the signers array parameter had to be empty:

let account = Test.serviceAccount()
let receiver = Test.createAccount()

let code = Test.readFile("../transactions/transfer_flow_tokens.cdc")
let tx = Test.Transaction(
    code: code,
    authorizers: [account.address],
    signers: [],
    arguments: [receiver.address, 1500.0]
)

let txResult = Test.executeTransaction(tx)

However, this special case adds some unnecessary complexity to developers, hence we always require the signers array parameter to contain the service account:

let account = Test.serviceAccount()
let receiver = Test.createAccount()

let code = Test.readFile("../transactions/transfer_flow_tokens.cdc")
let tx = Test.Transaction(
    code: code,
    authorizers: [account.address],
    signers: [account],
    arguments: [receiver.address, 1500.0]
)

let txResult = Test.executeTransaction(tx)

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