Fix two mistakes in next test instance classes (vercel#67633)

This is a quick follow-up from vercel#67601:

- avoid floating promise in `NextDevInstance#deleteFile`
- fix nesting in `NextInstance#stop`

Author: unstubbable

test/lib/next-modes/base.ts

@@ -394,9 +394,9 @@ export class NextInstance {
       this.childProcess.kill('SIGKILL')
       await exitPromise
       this.childProcess = undefined
+      this.isStopping = false
       require('console').log(`Stopped next server`)
     }
-    this.isStopping = false
   }
 
   public async destroy(): Promise<void> {

test/lib/next-modes/next-dev.ts

@@ -195,7 +195,7 @@ export class NextDevInstance extends NextInstance {
 
   public override async deleteFile(filename: string) {
     await this.handleDevWatchDelayBeforeChange(filename)
-    super.deleteFile(filename)
+    await super.deleteFile(filename)
     await this.handleDevWatchDelayAfterChange(filename)
   }
 }