Add Cash*Flow tests

Add tests for CashIssueFlow, CashPaymentFlow and CashExitFlow. While these were mostly covered by other tests
already, CashExistFlow was not, and any bugs would be harder to identify because they are mixed in with other
functionality (i.e. vault tests)

node/src/main/kotlin/net/corda/node/services/vault/NodeVaultService.kt

@@ -528,9 +528,11 @@ class NodeVaultService(private val services: ServiceHub, dataSourceProperties: P
      * @param amount the amount to gather states up to.
      * @throws InsufficientBalanceException if there isn't enough value in the states to cover the requested amount.
      */
+    // TODO: Merge this with the function in [AbstractConserveAmount]
     @Throws(InsufficientBalanceException::class)
     private fun gatherCoins(acceptableCoins: Collection<StateAndRef<Cash.State>>,
                             amount: Amount<Currency>): Pair<ArrayList<StateAndRef<Cash.State>>, Amount<Currency>> {
+        require(amount.quantity > 0) { "Cannot gather zero coins" }
         val gathered = arrayListOf<StateAndRef<Cash.State>>()
         var gatheredAmount = Amount(0, amount.token)
         for (c in acceptableCoins) {