[ticket/12341] Add tests for get_username_string()

PHPBB3-12341
Merge remote-tracking branch 'Elsensee/ticket/12341' into develop-olympus

* Elsensee/ticket/12341:
  [ticket/12341] Add tests for get_username_string()
[ticket/12318] Revert "Pin HHVM version to 3.0.0~precise."

This reverts commit f6614f2078740675ca0228b348500991888c2140 as the install
command started failing with "Version '3.0.0~precise' for 'hhvm' was not found"
in the meantime.

PHPBB3-12318
Merge remote-tracking branch 'bantu/ticket/12318' into develop-olympus

* bantu/ticket/12318:
  [ticket/12318] Revert "Pin HHVM version to 3.0.0~precise."