test: migrate from AVA to Node.js native test runner (#346)

AVA stores snapshots in a binary format (`.snap`), which produces no
meaningful diffs and bloats Git history. This replaces AVA with the
built-in `node:test` module, whose snapshot support generates
human-readable text files that are easy to diff and review in pull
requests.

The migration also replaces `@sinonjs/fake-timers` and `execa` with
Node.js built-ins (`node:test` mock timers and `node:child_process`),
removing three dev dependencies total.

### Changes

- **`tests/index.js`**: Rewritten to use `node:test` with a custom
snapshot serializer that renders strings with actual newlines. Uses
subtests for labeled `stderr`/`stdout` snapshots, and only snapshots
non-empty output.
- **`tests/main-repo-skew.test.js`**: Replace `@sinonjs/fake-timers`
with `mock.timers.enable()` from `node:test`.
- **`tests/README.md`**: Updated documentation to reflect `node --test`
and the new snapshot file.
- **`package.json`**: Remove `ava`, `@sinonjs/fake-timers`, and `execa`
from devDependencies. Update test script to `c8 --100 node --test
tests/index.js`.
- **`tests/index.js.snapshot`**: New text-based snapshot file replacing
binary `tests/snapshots/index.js.snap`.
- **`tests/snapshots/`**: Deleted.

All 22 test scenarios (66 subtests) pass with 100% code coverage.

Closes #344

---------

Co-authored-by: Copilot <[email protected]>
Co-authored-by: Copilot Autofix powered by AI <[email protected]>

tests/main-repo-skew.test.js

@@ -1,6 +1,6 @@
-import { test } from "./main.js";
+import { mock } from "node:test";
 
-import { install } from "@sinonjs/fake-timers";
+import { test } from "./main.js";
 
 // Verify `main` retry when the clock has drifted.
 await test((mockPool) => {
@@ -11,7 +11,7 @@ await test((mockPool) => {
   const mockInstallationId = "123456";
   const mockAppSlug = "github-actions";
 
-  install({ now: 0, toFake: ["Date"] });
+  mock.timers.enable({ apis: ["Date"], now: 0 });
 
   mockPool
     .intercept({
@@ -59,4 +59,6 @@ await test((mockPool) => {
       };
     })
     .times(2);
+}).finally(() => {
+  mock.timers.reset();
 });