chore: import upstream snapshot with attribution

ai/references/tests.md

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+## Running Tests
+
+We use vitest exclusively for testing. To execute tests for a particular workspace, run the following command:
+
+```bash
+pnpm run test --filter webapp
+```
+
+Prefer running tests on a single file (and first cding into the directory):
+
+```bash
+cd apps/webapp
+pnpm run test ./src/components/Button.test.ts
+```
+
+If you are cd'ing into a directory, you may have to build dependencies first:
+
+```bash
+pnpm run build --filter webapp
+cd apps/webapp
+pnpm run test ./src/components/Button.test.ts
+```
+
+## Writing Tests
+
+We use vitest for testing. We almost NEVER mock anything. Start with a top-level "describe", and have multiple "it" statements inside of it.
+
+New test files should be placed right next to the file being tested. For example:
+
+- Source file: `./src/services/MyService.ts`
+- Test file: `./src/services/MyService.test.ts`
+
+When writing anything that needs redis or postgresql, we have some internal "testcontainers" that are used to spin up a local instance, redis, or both.
+
+redisTest:
+
+```typescript
+import { redisTest } from "@internal/testcontainers";
+import { createRedisClient } from "@internal/redis";
+
+describe("redisTest", () => {
+  redisTest("should use redis", async ({ redisOptions }) => {
+    const redis = createRedisClient(redisOptions);
+
+    await redis.set("test", "test");
+    const result = await redis.get("test");
+    expect(result).toEqual("test");
+  });
+});
+```
+
+postgresTest:
+
+```typescript
+import { postgresTest } from "@internal/testcontainers";
+
+describe("postgresTest", () => {
+  postgresTest("should use postgres", async ({ prisma }) => {
+    // prisma is an instance of PrismaClient
+  });
+});
+```
+
+containerTest:
+
+```typescript
+import { containerTest } from "@internal/testcontainers";
+
+describe("containerTest", () => {
+  containerTest("should use container", async ({ prisma, redisOptions }) => {
+    // container has both prisma and redis
+  });
+});
+```
+
+## Dos and Dont's
+
+- Do not mock anything.
+- Do not use mocks in tests.
+- Do not use spies in tests.
+- Do not use stubs in tests.
+- Do not use fakes in tests.
+- Do not use sinon in tests.
+- Structure each test with a setup, action, and assertion style.
+- Feel free to write long test names.
+- If there is any randomness in the code under test, use `seedrandom` to make it deterministic by allowing the caller to provide a seed.