Added unit test for TiersRepository

We were completely missing tests for this, and the new logic pushed us under
the coverage threshold.

ghost/core/test/unit/server/services/tiers/TierRepository.test.js

@@ -0,0 +1,54 @@
+const assert = require('assert');
+const sinon = require('sinon');
+const {Product: ProductModel} = require('../../../../../core/server/models/product');
+const TierRepository = require('../../../../../core/server/services/tiers/TierRepository');
+const {Tier} = require('@tryghost/tiers');
+
+describe('TierRepository', function () {
+    after(function () {
+        sinon.restore();
+    });
+
+    it('Can do basic functionality', async function () {
+        const DomainEvents = {
+            dispatch: sinon.stub()
+        };
+        sinon.stub(ProductModel, 'findAll').resolves([]);
+        sinon.stub(ProductModel, 'findOne').resolves([]);
+        sinon.stub(ProductModel, 'edit').resolves();
+        sinon.stub(ProductModel, 'add').resolves();
+
+        const repository = new TierRepository({
+            DomainEvents,
+            ProductModel
+        });
+
+        await repository.init();
+
+        const tier = await Tier.create({
+            name: 'Test',
+            slug: 'test-tier',
+            type: 'paid',
+            status: 'active',
+            currency: 'USD',
+            monthlyPrice: 5,
+            yearlyPrice: 50
+        });
+
+        await repository.save(tier);
+
+        assert(ProductModel.add.calledOnce);
+
+        const result = await repository.getById(tier.id);
+
+        assert(ProductModel.findOne.notCalled);
+
+        assert(result);
+
+        result.name = 'New Name';
+
+        await repository.save(tier);
+
+        assert(ProductModel.edit.calledOnce);
+    });
+});