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

PHPBB3-11541
[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()
[ticket/11545] Remove DIRECTORY_SEPARATOR dependency from is_absolute

The given path is an absolute path in general, just not on our current system.

PHPBB3-11545
Merge remote-tracking branch 'nickvergessen/ticket/11545' into develop-olympus

* nickvergessen/ticket/11545:
  [ticket/11545] Remove DIRECTORY_SEPARATOR dependency from is_absolute