[ticket/9341] Correctly named template vars

PHPBB3-9341
[ticket/9341] Use U_PREVIOUS and U_NEXT in template files

PHPBB3-9341
[ticket/9341] Move create_topic and post into functional test case

create_topic and create_post are moved into functional test case
so that they can be used by other tests as well

PHPBB3-9341
[ticket/9341] Add tests for checking Next and Previous template vars

PHPBB3-9341
[ticket/9341] remove PREV_PAGE tpl var used for backward compatability

PHPBB3-9341
[ticket/9341] Follow the Next/Prev link in tests
follow the next/prev link and then assert if the page contains
its last post or not

PHPBB3-9341
[ticket/9341] Assert that page doesnt contain next or prev page posts

PHPBB3-9341
Merge remote-tracking branch 'dhruvgoel92/ticket/9341' into develop

* dhruvgoel92/ticket/9341:
  [ticket/9341] Assert that page doesnt contain next or prev page posts
  [ticket/9341] Follow the Next/Prev link in tests follow the next/prev link and then assert if the page contains its last post or not
  [ticket/9341] remove PREV_PAGE tpl var used for backward compatability
  [ticket/9341] Add tests for checking Next and Previous template vars
  [ticket/9341] Move create_topic and post into functional test case
  [ticket/9341] Use U_PREVIOUS and U_NEXT in template files
  [ticket/9341] Correctly named template vars