chore(extension): add test (#842)

* On Linux headed mode under xvfb-run fails to properly launch the
process. It works fine without xvfb-run, we don't have environment for
that on CI, so run on macOS instead.
* Node v18.20.8 stalls on `const uuid = crypto.randomUUID();`, so use
v20 for the extension tests.

tests/fixtures.ts

@@ -191,7 +191,7 @@ async function createTransport(args: string[], mcpMode: TestOptions['mcpMode'],
   const transport = new StdioClientTransport({
     command: 'node',
     args: [path.join(path.dirname(__filename), '../cli.js'), ...args],
-    cwd: path.join(path.dirname(__filename), '..'),
+    cwd: path.dirname(test.info().config.configFile!),
     stderr: 'pipe',
     env: {
       ...process.env,