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 <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

tests/README.md

@@ -2,14 +2,14 @@
 
 Add one test file per scenario. You can run them in isolation with:
 
-```bash
+```
 node tests/post-token-set.test.js
 ```
 
-All tests are run together in [tests/index.js](index.js), which can be executed with ava
+All tests are run together in [tests/index.js](index.js), which can be executed with Node's built-in test runner
 
 ```
-npx ava tests/index.js
+node --test tests/index.js
 ```
 
 or with npm
@@ -20,7 +20,13 @@ npm test
 
 ## How the tests work
 
-The output from the tests is captured into a snapshot ([tests/snapshots/index.js.md](snapshots/index.js.md)). It includes all requests sent by our scripts to verify it's working correctly and to prevent regressions.
+The output from the tests is captured into a snapshot ([tests/index.js.snapshot](index.js.snapshot)). It includes all requests sent by our scripts to verify it's working correctly and to prevent regressions.
+
+To update snapshots after an intentional change:
+
+```
+node --test --test-update-snapshots tests/index.js
+```
 
 ## How to add a new test