Improve handling of failed nextBuild() calls in test suite (#63717)

## What?

Currently `next build` with Turbopack fails to run in certain tests (as
expected), the problem with that is that the `afterAll()` calls assume
the server was started, which doesn't happen when the build fails, and
then causes a timeout that can't be reported if an error happens. This
change ensures the error around `server['__app']` doesn't happen.

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Closes NEXT-2933

test/lib/next-test-utils.ts

@@ -529,6 +529,9 @@ export async function killApp(
   instance?: ChildProcess,
   signal: NodeJS.Signals | number = 'SIGKILL'
 ) {
+  if (!instance) {
+    return
+  }
   if (
     instance?.pid &&
     instance.exitCode === null &&
@@ -558,7 +561,10 @@ export async function startApp(app: NextServer) {
   return server
 }
 
-export async function stopApp(server: http.Server) {
+export async function stopApp(server: http.Server | undefined) {
+  if (!server) {
+    return
+  }
   if (server['__app']) {
     await server['__app'].close()
   }

test/production/bfcache-routing/index.test.ts

@@ -22,9 +22,7 @@ describe('bfcache-routing', () => {
   })
 
   afterAll(() => {
-    if (app) {
-      stopApp(app)
-    }
+    stopApp(app)
   })
 
   it('should not suspend indefinitely when page is restored from bfcache after an mpa navigation', async () => {