[ticket/11480] PM : "Unknown folder" returned when inbox folder is full

https://tracker.phpbb.com/browse/PHPBB3-11480

PHPBB3-11480
[ticket/11480] Add functionnal test

PHPBB3-11480
[ticket/11480] Fix typos

PHPBB3-11480
[ticket/11480] Fix the double spaces in phpbb_functional_test_case

PHPBB3-11480
[ticket/11480] Move the test into get_folder()

PHPBB3-11480
Merge pull request #2350 from Nicofuma/ticket/11480

[ticket/11480] PM : "Unknown folder" returned when inbox folder is full

* Nicofuma/ticket/11480:
  [ticket/11480] Move the test into get_folder()
  [ticket/11480] Fix the double spaces in phpbb_functional_test_case
  [ticket/11480] Fix typos
  [ticket/11480] Add functionnal test
  [ticket/11480] PM : "Unknown folder" returned when inbox folder is full