Can anyone explain me in detail or send me some link that will explain How to create test user account for inapp purchase?
You need to login into your itunesconnect account. then choose Members menu (in the bottom left) add a test account.
https://developer.apple.com/library/ios/documentation/LanguagesUtilities/Conceptual/iTunesConnect_Guide/Chapters/SettingUpUserAccounts.html#//apple_ref/doc/uid/TP40011225-CH25-SW10
Can you see this in your iTunes Connect page?
If not, make sure you have information in these two sections ready: