Rewrite 2 out of 3 Auction tests to use mockist testing

This changes how the tests work as an example of behavior verification,
as opposed to state verification. The third test is still not ported,
see: sinonjs/sinon#1501

code/in-search-of-a-unit/test/Auction.test.ts

@@ -4,6 +4,7 @@ import { Money } from "../src/Money";
 import * as uuid from 'uuid';
 import { Offer } from "../src/Offer";
 import * as assert from "assert";
+import * as sinon from "sinon";
 
 function testOffer(amount: number, currency = 'PLN', user?: string): Offer {
     return {
@@ -26,25 +27,36 @@ function stuffAuction(auction: Auction, count: number) {
 describe('Auction', function() {
     it('should immediately reject an offer that could not possibly win', async function() {
         const notifier = new EmailNotifier();
+        const mockNotifier = sinon.mock(notifier);
         const sale = new Auction('PLN', 5, notifier);
         // Initially, everybody goes into auction at 10 PLN:
         const initialOffers = stuffAuction(sale, 5);
         // Subsequent offers for the same amount should be rejected:
-        const sixthOfferResult = await sale.makeOffer(testOffer(10));
-        assert.strictEqual(sixthOfferResult, false);
+        const sixthOffer = testOffer(10);
+        mockNotifier.expects('notifyUser').once().withArgs(sixthOffer.madeByUserID, {
+            eventType: 'outbid',
+            offer: sixthOffer
+        }).resolves();
+        const sixthOfferResult = await sale.makeOffer(sixthOffer);
+        mockNotifier.verify();
     });
     it('should replace the last offer if a better one comes in', async function() {
         const notifier = new EmailNotifier();
+        const mockNotifier = sinon.mock(notifier);
         const sale = new Auction('PLN', 5, notifier);
         const initialOffers = stuffAuction(sale, 5);
         // A new offer should annul the last one made:
         const lastOffer = sale.getQualifyingOffers().pop()!;
         const betterOffer = testOffer(15);
+        mockNotifier.expects('notifyUser').once().withArgs(lastOffer.madeByUserID, {
+            eventType: 'outbid',
+            offer: lastOffer
+        }).resolves();
         await sale.makeOffer(betterOffer);
-        assert.ok(sale.getQualifyingOffers().includes(betterOffer));
-        assert.ok(!sale.getQualifyingOffers().includes(lastOffer));
+        mockNotifier.verify();
     });
     it('should select the winners after some offers are replaced', async function() {
+        // TODO: Rewrite this test to use behavior verification as well!
         const notifier = new EmailNotifier();
         const sale = new Auction('PLN', 5, notifier);
         const initialOffers = stuffAuction(sale, 5);

code/package.json

@@ -19,6 +19,7 @@
     "@types/big.js": "^6.0.0",
     "@types/mocha": "^8.0.4",
     "@types/node": "^14.14.7",
+    "@types/sinon": "^9.0.8",
     "@types/uuid": "^8.3.0",
     "ts-node": "^9.0.0",
     "typescript": "^4.0.5"