chore: add test mapping script (#63757)

### What?

This adds a script that first gets a list of changed files compared to
our base branch (`canary`) in PRs and identifies related test files by
looking them up from a `related-tests-manifest.json`, then runs them.

### Why?

We would like to be able to run related E2E tests when certain files
change, so we can test deployment stability.

NOTE: We still want to run all E2E tests on PRs, but this script can be
used to also test deployments for hot paths. Since these tests can take
up to [3-4
hours](https://github.com/vercel/next.js/actions/workflows/test_e2e_deploy.yml),
we aim to run a subset of these tests first.


Closes NEXT-2947

scripts/run-related-test.mjs

@@ -0,0 +1,54 @@
+/**
+ * This script reads the 'test/realted-tests-manifest.json' file, which contains a list of test globs that maps source code files to test files.
+ * The script is used to find all related test cases for a given code change.
+ * Usage: `node scripts/run-related-test.mjs <path1> <path2> ...`
+ */
+
+import { promisify } from 'node:util'
+import { exec as execOrg } from 'node:child_process'
+import { readFile } from 'node:fs/promises'
+import { join } from 'node:path'
+
+const exec = promisify(execOrg)
+
+/**
+ * Get all changed files from git under the packages directory
+ * @returns {Promise<string[]>} - List of changed files
+ */
+async function getChangedFilesFromPackages(baseBranch = 'canary') {
+  await exec('git config --global --add safe.directory /work')
+  await exec(`git remote set-branches --add origin ${baseBranch}`)
+  await exec(`git fetch origin ${baseBranch} --depth=20`)
+  const { stdout } = await exec(`git diff --name-only ${baseBranch}`)
+  return stdout
+    .trim()
+    .split('\n')
+    .filter((line) => line.startsWith('packages/'))
+}
+
+export async function getRelatedTests(args = []) {
+  const relatedTestsManifest = join(
+    process.cwd(),
+    'test',
+    'related-tests-manifest.json'
+  )
+  /** @type {Record<string, string[]>} */
+  const manifest = JSON.parse(
+    await readFile(relatedTestsManifest, 'utf-8').catch(() => '{}')
+  )
+  const tests = []
+  const paths = args.length ? args : await getChangedFilesFromPackages()
+  for (const path of paths) {
+    const relatedTestsKey = Object.keys(manifest).find((key) =>
+      path.startsWith(key)
+    )
+    if (relatedTestsKey) {
+      const lines = manifest[relatedTestsKey]
+      tests.push(...lines)
+    }
+  }
+
+  return Array.from(new Set(tests))
+}
+
+// console.log(await getRelatedTests(process.argv.slice(2)))

test/related-tests-manifest.json

@@ -0,0 +1,3 @@
+{
+  "packages/next/src/server": ["e2e/app-dir/ppr-*"]
+}