[ticket/12224] Add template wrapper method to assign block arrays

Add one more wrapper template method for the function assign_block_vars() which
takes a 2-dimensional array as a parameter
and calls assign_block_vars() in a loop to assign the whole block loop at once.
This can make the core a little bit more expendable from the point of
developing extensions as it allows to pass the data to events before
it's being assigned to template.

PHPBB3-12224
[ticket/12224] Add assign_block_vars_array() test.

Add tests, also change return value to null.

PHPBB3-12224
Merge remote-tracking branch 'rxu3/ticket/12224' into develop

* rxu3/ticket/12224:
  [ticket/12224] Add assign_block_vars_array() test.
  [ticket/12224] Add template wrapper method to assign block arrays