feat(tests): add basic arithmetic functions and corresponding tests

- Implemented `add` and `subtract` functions in multiple packages (web, ui, utils).
- Created unit tests for these functions using Vitest in respective test files.
- Updated turbo.json to include a new test pipeline for coverage and watch mode.

packages/utils/src/testing-example/add.ts

@@ -0,0 +1 @@
+export const add = (a: number, b: number) => a + b;

packages/utils/src/testing-example/subtract.ts

@@ -0,0 +1 @@
+export const subtract = (a: number, b: number) => a - b;

packages/utils/src/testing-example/utils-testing-example.test.ts

@@ -0,0 +1,11 @@
+import { expect, test } from 'vitest';
+import { add } from './add';
+import { subtract } from './subtract';
+
+test('adds 1 + 2 to equal 3', () => {
+  expect(add(1, 2)).toBe(3);
+});
+
+test('subtracts 2 - 1 to equal 1', () => {
+  expect(subtract(2, 1)).toBe(1);
+});