Due to the usage of `__DIR__` for the file includes the tests can't
be ran on systems with PHP < 5.3. Replace all occurances of
`__DIR__` with `dirname(__FILE__)`.
PHPBB3-10011
Rename $unicode to $unicode_support, pass in $use_unicode defaulting to true.
In unit tests we can now pass in $use_unicode as false and also test the code
path that is taken when PCRE does not support unicode.
PHPBB3-9933
