test(nexttestsetup): teardown nextinstance gracefully (#55144)

### What?

https://vercel.slack.com/archives/C04KC8A53T7/p1694190462958199

I realized we throws an error attempt to destory against undefined instance when `beforeAll` fails in any reason. This is quite redundant error since if next instance doesn't exist, likely there's an upstream test failures already and destory against undefined error is not useful to debug anyway.

PR replaces `nextTestSetup`'s teardown first, for the remaining places calling `next.destory` explicitly will be amended later.

test/lib/e2e-utils.ts

@@ -233,7 +233,12 @@ export function nextTestSetup(
       next = await createNext(options)
     })
     afterAll(async () => {
-      await next.destroy()
+      // Gracefully destroy the instance if `createNext` success.
+      // If next instnace is not available, it's likely beforeAll hook failed and unnecessariliy throws another error
+      // by attempting to destroy on undefined.
+      if (next) {
+        await next.destroy()
+      }
     })
   }
 

test/lib/next-test-utils.ts

@@ -536,7 +536,9 @@ export async function killProcess(pid: number): Promise<void> {
 
 // Kill a launched app
 export async function killApp(instance: ChildProcess) {
-  await killProcess(instance.pid)
+  if (instance && instance.pid) {
+    await killProcess(instance.pid)
+  }
 }
 
 export async function startApp(app: NextServer) {