Improve stakeNeuron test coverage (#743)

# Motivation

I want to refactor the memo to neuron (sub)account calculation to make
it available to clients.
But currently the calculated account identifier is not tested.
So before I start changing that code I want to make sure it's tested.

# Changes

1. Mock the randomly generated memo.
2. Expect specific values in the calls to `transfer` and
`manage_neuron`.

# Tests

PR is test-only. Tests pass.

# Todos

- [ ] Add entry to changelog (if necessary).
not necessary

packages/nns/src/governance.canister.spec.ts

@@ -1,5 +1,9 @@
 import { ActorSubclass, AnonymousIdentity } from "@dfinity/agent";
-import { InvalidAccountIDError, LedgerCanister } from "@dfinity/ledger-icp";
+import {
+  AccountIdentifier,
+  InvalidAccountIDError,
+  LedgerCanister,
+} from "@dfinity/ledger-icp";
 import { Principal } from "@dfinity/principal";
 import { InvalidPercentageError } from "@dfinity/utils";
 import { mock } from "jest-mock-extended";
@@ -46,6 +50,24 @@ import {
   UpdateCanisterSettings,
 } from "./types/governance_converters";
 
+const nextRandomBytes: number[] = [];
+
+jest.mock("randombytes", () => ({
+  __esModule: true,
+  default: function (n: number) {
+    const nums: number[] = [];
+    for (let i = 0; i < n; i++) {
+      const nextByte = nextRandomBytes.shift();
+      if (nextByte !== undefined) {
+        nums.push(nextByte);
+      } else {
+        nums.push(Math.floor(Math.random() * 0x100));
+      }
+    }
+    return new Uint8Array(nums);
+  },
+}));
+
 const unexpectedGovernanceError: GovernanceErrorDetail = {
   error_message: "Error updating neuron",
   error_type: 0,
@@ -197,17 +219,52 @@ describe("GovernanceCanister", () => {
         jest.fn().mockResolvedValue(BigInt(1)),
       );
 
+      const stake = 100_000_000n;
+      const controller = Principal.fromText("2amoo-gaqe4-3ekjz-wiu");
+
+      nextRandomBytes.push(123, 4, 56, 7, 8, 90, 1, 234, 5, 67);
+      const expectedMemo = 8864271569428021738n;
+      const expectedIcpAccount = AccountIdentifier.fromHex(
+        "911c1b8826981b7126dc62cf176f30550eefee5d31e2071143ca17e727b251e5",
+      );
+
+      expect(mockLedger.transfer).toBeCalledTimes(0);
+      expect(service.manage_neuron).toBeCalledTimes(0);
+
       const governance = GovernanceCanister.create({
         certifiedServiceOverride: service,
       });
       const response = await governance.stakeNeuron({
-        stake: BigInt(100_000_000),
-        principal: new AnonymousIdentity().getPrincipal(),
+        stake,
+        principal: controller,
         ledgerCanister: mockLedger,
       });
 
-      expect(mockLedger.transfer).toBeCalled();
-      expect(service.manage_neuron).toBeCalled();
+      expect(mockLedger.transfer).toBeCalledTimes(1);
+      expect(mockLedger.transfer).toBeCalledWith({
+        amount: stake,
+        memo: expectedMemo,
+        to: expectedIcpAccount,
+      });
+      expect(service.manage_neuron).toBeCalledTimes(1);
+      expect(service.manage_neuron).toBeCalledWith({
+        command: [
+          {
+            ClaimOrRefresh: {
+              by: [
+                {
+                  MemoAndController: {
+                    controller: [controller],
+                    memo: expectedMemo,
+                  },
+                },
+              ],
+            },
+          },
+        ],
+        id: [],
+        neuron_id_or_subaccount: [],
+      });
       expect(response).toEqual(neuronId);
     });