Improve server actions sourcemap test (#76129)

## What?

Improves the check for browser sourcemaps by reading all sourcemap files
from the `.next/static` folder instead of trying to locate a specific
value. The sensitive value that is being checked should not exist in any
of the sourcemap files so it's safer to check them all. This also
ensures that if for example the underlying bundling heuristics change
the check will still fail. In this case it was failing on the path with
Turbopack, instead of on the value. This ensures it checks the value.

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---------

Co-authored-by: Sebastian "Sebbie" Silbermann <sebastian.silbermann@vercel.com>

test/e2e/app-dir/actions/app-action.test.ts

@@ -9,6 +9,7 @@ import {
 } from 'next-test-utils'
 import type { Page, Request, Response, Route } from 'playwright'
 import fs from 'fs-extra'
+import nodeFs from 'fs'
 import { join } from 'path'
 
 const GENERIC_RSC_ERROR =
@@ -919,19 +920,24 @@ describe('app-dir action handling', () => {
 
   if (isNextStart) {
     it('should not expose action content in sourcemaps', async () => {
-      const sourcemap = (
-        await fs.readdir(
-          join(next.testDir, '.next', 'static', 'chunks', 'app', 'client')
+      // We check all sourcemaps in the `static` folder for sensitive information given that chunking
+      const sourcemaps = nodeFs
+        .readdirSync(join(next.testDir, '.next', 'static'), {
+          recursive: true,
+          encoding: 'utf8',
+        })
+        .filter((f) => f.endsWith('.js.map'))
+        .map((f) =>
+          nodeFs.readFileSync(join(next.testDir, '.next', 'static', f), {
+            encoding: 'utf8',
+          })
         )
-      ).find((f) => f.endsWith('.js.map'))
 
-      expect(sourcemap).toBeDefined()
+      expect(sourcemaps).not.toBeEmpty()
 
-      expect(
-        await next.readFile(
-          join('.next', 'static', 'chunks', 'app', 'client', sourcemap)
-        )
-      ).not.toContain('this_is_sensitive_info')
+      for (const sourcemap of sourcemaps) {
+        expect(sourcemap).not.toContain('this_is_sensitive_info')
+      }
     })
   }