test(next-dev): hard fail if next-dev runs with turbopack intenral fl…

…ag (#51308)

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### What?

WEB-1183

Context:
https://vercel.slack.com/archives/C04KC8A53T7/p1686768648161459?thread_ts=1686763527.218549&cid=C04KC8A53T7

We'd like to avoid false positives to the test cases if test supposed to
run turbopack (having __INTERNAL_ flags) but somehow test fixture does
not run turbopack correctly.

packages/next/src/cli/next-dev.ts

@@ -295,6 +295,12 @@ const nextDev: CliCommand = async (argv) => {
   } else {
     let cleanupFns: (() => Promise<void> | void)[] = []
     const runDevServer = async () => {
+      if (process.env.__INTERNAL_NEXT_DEV_TEST_TURBO_DEV) {
+        require('console').error(
+          `[ERROR]: Incorrect mode: turbopack test mode is force enabled, shouldn't run dev server`
+        )
+        process.exit(1)
+      }
       const oldCleanupFns = cleanupFns
       cleanupFns = []
       await Promise.allSettled(oldCleanupFns.map((fn) => fn()))