If a database is unable to nest transactions the dbal should implement
sql_buffer_nested_transactions to signal that buffering of the outer
results and closing of the outer transaction is required to open the
inner transaction.
PHPBB3-9685
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
