test: bind fixture servers to os-assigned ports

The test:* scripts run in parallel on CI, so a pre-picked free port could
be claimed by another suite before the bind (EADDRINUSE). The cdn server
now starts first on port 0 — 404ing until its dist exists — so its real
port can be baked into assetsBase. Also type the file:// image probe.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

__tests__/base/relative-file.test.ts

@@ -31,7 +31,9 @@ describe('relative base opened over file://', () => {
     )
     expect(fontFamily).toContain('Inter')
     const logoLoaded = await t.page.evaluate(
-      () => document.querySelector('img[alt="logo again"]')!.naturalWidth
+      () =>
+        document.querySelector<HTMLImageElement>('img[alt="logo again"]')!
+          .naturalWidth
     )
     expect(logoLoaded).toBe(1)
   })

__tests__/base/vitestGlobalSetup.ts

@@ -1,10 +1,10 @@
 import { spawnSync } from 'node:child_process'
 import { readFile } from 'node:fs/promises'
 import { createServer, type Server } from 'node:http'
+import type { AddressInfo } from 'node:net'
 import { extname, join, resolve } from 'node:path'
 import { fileURLToPath } from 'node:url'
 
-import getPort from 'get-port'
 import { chromium, type BrowserServer } from 'playwright-chromium'
 
 import { ALT_PREFIX, SUB_PREFIX } from './constants'
@@ -23,8 +23,9 @@ const types: Record<string, string> = {
   '.zip': 'application/zip'
 }
 
+// listens on an os-assigned port (the other suites run in parallel on CI,
+// so a pre-picked "free" port can be taken before we bind it)
 function serveStatic(
-  port: number,
   mounts: [prefix: string, root: string][],
   cors: boolean
 ): Promise<Server> {
@@ -48,18 +49,19 @@ function serveStatic(
     res.writeHead(404)
     res.end('not found')
   })
-  return new Promise((r) => server.listen(port, () => r(server)))
+  return new Promise((r) => server.listen(0, () => r(server)))
 }
 
+const portOf = (server: Server) => (server.address() as AddressInfo).port
+
 let browserServer: BrowserServer
 let servers: Server[] = []
 
 export async function setup() {
-  const [subPort, pagesPort, cdnPort] = await Promise.all([
-    getPort(),
-    getPort(),
-    getPort()
-  ])
+  // the cdn server starts before its dist exists (requests just 404 until
+  // the build lands) so the real port can be baked into assetsBase
+  const cdnServer = await serveStatic([['/', dist('cdn')]], true)
+  const cdnPort = portOf(cdnServer)
 
   // each flavor builds in its own process: the markdown renderer is a
   // process-wide singleton, so sequential in-process builds would leak the
@@ -82,15 +84,14 @@ export async function setup() {
   servers = [
     // one relative-base build mounted at two unrelated prefixes
     await serveStatic(
-      subPort,
       [
         [SUB_PREFIX, dist('relative')],
         [ALT_PREFIX, dist('relative')]
       ],
       false
     ),
-    await serveStatic(pagesPort, [['/', dist('cdn')]], false),
-    await serveStatic(cdnPort, [['/', dist('cdn')]], true)
+    await serveStatic([['/', dist('cdn')]], false),
+    cdnServer
   ]
 
   browserServer = await chromium.launchServer({
@@ -101,8 +102,8 @@ export async function setup() {
   })
 
   process.env['WS_ENDPOINT'] = browserServer.wsEndpoint()
-  process.env['SUB_PORT'] = String(subPort)
-  process.env['PAGES_PORT'] = String(pagesPort)
+  process.env['SUB_PORT'] = String(portOf(servers[0]!))
+  process.env['PAGES_PORT'] = String(portOf(servers[1]!))
   process.env['VP_CDN_PORT'] = String(cdnPort)
 }