Add cursor rules for writing tests

.cursor/rules/building-bun.mdc

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+---
+description: How to build Bun
+globs: 
+---
+# How to build Bun
+
+## CMake
+
+Bun is built using CMake, which you can find in `CMakeLists.txt` and in the `cmake/` directory.
+
+* `CMakeLists.txt`
+* `cmake/`
+  * `Globals.cmake` - macros and functions used by all the other files
+  * `Options.cmake` - build options for configuring the build (e.g. debug/release mode)
+  * `CompilerFlags.cmake` - compiler and linker flags used by all the targets
+  * `tools/` - setup scripts for various build tools (e.g. llvm, zig, webkit, rust, etc.)
+  * `targets/` - targets for bun and its dependencies (e.g. brotli, boringssl, libuv, etc.)
+
+## How to
+
+There are `package.json` scripts that make it easy to build Bun without calling CMake directly, for example:
+
+```sh
+bun run build # builds a debug build: `build/debug/bun-debug`
+bun run build:release # builds a release build: `build/release/bun`
+bun run build:assert # builds a release build with debug assertions: `build/assert/bun`
+```

.cursor/rules/writing-tests.mdc

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+---
+description: Writing tests for Bun
+globs: 
+---
+# Writing tests for Bun
+
+## Where tests are found
+
+You'll find all of Bun's tests in the `test/` directory.
+
+* `test/`
+  * `cli/` - CLI command tests, like `bun install` or `bun init`
+  * `js/` - JavaScript & TypeScript tests
+    * `bun/` - `Bun` APIs tests, seperated by category, for example: `glob/` for `Bun.Glob` tests
+    * `node/` - Node.js module tests, seperated by module, for example: `assert/` for `node:assert` tests
+      * `test/` - Vendored Node.js tests, taken from the Node.js repository (does not conform to Bun's test style)
+    * `web/` - Web API tests, seperated by category, for example: `fetch/` for `Request` and `Response` tests
+    * `third_party/` - npm package tests, to validate that basic usage works in Bun
+  * `napi/` - N-API tests
+  * `v8/` - V8 C++ API tests
+  * `bundler/` - Bundler, transpiler, CSS, and `bun build` tests
+  * `regression/issue/[number]` - Regression tests, always make one when fixing a particular issue
+
+## How tests are written
+
+Bun's tests are written as JavaScript and TypeScript files with the Jest-style APIs, like `test`, `describe`, and `expect`. They are tested using Bun's own test runner, `bun test`. 
+
+```js
+import { describe, test, expect } from "bun:test";
+import assert, { AssertionError } from "assert";
+
+describe("assert(expr)", () => {
+  test.each([true, 1, "foo"])(`assert(%p) does not throw`, expr => {
+    expect(() => assert(expr)).not.toThrow();
+  });
+
+  test.each([false, 0, "", null, undefined])(`assert(%p) throws`, expr => {
+    expect(() => assert(expr)).toThrow(AssertionError);
+  });
+});
+```
+
+## Testing conventions
+
+* See `test/harness.ts` for common test utilities and helpers
+* Be rigorous and test for edge-cases and unexpected inputs
+* Use data-driven tests, e.g. `test.each`, to reduce boilerplate when possible
+* When you need to test Bun as a CLI, use the following pattern:
+
+```js
+import { test, expect } from "bun:test";
+import { spawn } from "bun";
+import { bunExe, bunEnv } from "harness";
+
+test("bun --version", async () => {
+  const { exited, stdout: stdoutStream, stderr: stderrStream } = spawn({
+    cmd: [bunExe(), "--version"],
+    env: bunEnv,
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+  const [ exitCode, stdout, stderr ] = await Promise.all([
+    exited,
+    new Response(stdoutStream).text(),
+    new Response(stderrStream).text(),
+  ]);
+  expect({ exitCode, stdout, stderr }).toMatchObject({
+    exitCode: 0,
+    stdout: expect.stringContaining(Bun.version),
+    stderr: "",
+  });
+});
+```
+
+## Before writing a test
+
+* If you are fixing a bug, write the test first and make sure it fails (as expected) with the canary version of Bun
+* If you are fixing a Node.js compatibility bug, create a throw-away snippet of code and test that it works as you expect in Node.js, then that it fails (as expected) with the canary version of Bun
+* When the expected behaviour is ambigious, defer to matching what happens in Node.js
+* Always attempt to find related tests in an existing test file before creating a new test file
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