[ticket/12175] Fix upload test for empty file

PHPBB3-12175
[ticket/12175] Fix test for uploading a valid file

PHPBB3-12175
[ticket/12175] Do not use admin account for file size limit test

PHPBB3-12175
Merge pull request #2022 from nickvergessen/ticket/12175

Ticket/12175 Fix and run functional upload tests

* nickvergessen/ticket/12175:
  [ticket/12175] Do not use admin account for file size limit test
  [ticket/12175] Fix test for uploading a valid file
  [ticket/12175] Fix upload test for empty file