chore: fix live-preview tests against prod (#9122)

Live preview e2e tests had no CSS when tested against prod.

For all our other tests, we have a separate test/app directory that
imports CSS. Otherwise, the root-level /app directory is used.

For live-preview, we currently always run against test/live-preview/app,
that has no CSS import.

This PR adds a new test/live-preview/prod/app directory that imports CSS
and is used when we run tests against prod.

In order for this to work, I had to make import map generation smarter

test/generateImportMap.ts

@@ -4,6 +4,7 @@ const [testConfigDir] = process.argv.slice(2)
 
 import type { SanitizedConfig } from 'payload'
 
+import fs from 'fs'
 import { generateImportMap } from 'payload'
 import { fileURLToPath } from 'url'
 
@@ -21,11 +22,24 @@ async function run() {
 
     const config: SanitizedConfig = await (await import(pathWithConfig)).default
 
-    process.env.ROOT_DIR =
-      testConfigDir === 'live-preview' || testConfigDir === 'admin-root'
-        ? testDir
-        : path.resolve(dirname, '..')
+    let rootDir = ''
+    if (testConfigDir === 'live-preview' || testConfigDir === 'admin-root') {
+      rootDir = testDir
+      if (process.env.PAYLOAD_TEST_PROD === 'true') {
+        // If in prod mode, there may be a testSuite/prod folder. If so, use that as the rootDir
+        const prodDir = path.resolve(testDir, 'prod')
+        try {
+          fs.accessSync(prodDir, fs.constants.F_OK)
+          rootDir = prodDir
+        } catch (err) {
+          // Swallow err - no prod folder
+        }
+      }
+    } else {
+      rootDir = path.resolve(dirname, '..')
+    }
 
+    process.env.ROOT_DIR = rootDir
     await generateImportMap(config, { log: true, force: true })
   }
 }