[ticket/11540] Add unit tests for is_absolute()

PHPBB3-11540
[ticket/11541] Add unit tests for style_select()

PHPBB3-11541
Merge remote-tracking branch 'nickvergessen/ticket/11540' into develop-olympus

* nickvergessen/ticket/11540:
  [ticket/11540] Add unit tests for is_absolute()
[ticket/11546] Fix is_absolute() throws E_NOTICE for empty string

PHPBB3-11546
Merge remote-tracking branch 'nickvergessen/ticket/11546' into develop-olympus

* nickvergessen/ticket/11546:
  [ticket/11546] Fix is_absolute() throws E_NOTICE for empty string
Merge remote-tracking branch 'nickvergessen/ticket/11541' into develop-olympus

* nickvergessen/ticket/11541:
  [ticket/11541] Add unit tests for style_select()