[ticket/12963] Fix the list of migrations in database_update.php

Without this patch the finder grab all the class available in the ext
folder and not only the migrations. This change is backported for the one
done before to the cli tool db:migrate. (see the commit
3420f8f3201ac337434f73ee00bda6df7b378212)

PHPBB3-12963
[ticket/12963] Don't use static var in \extension\base\get_migration_file_list

The static var was global to all instance of \phpbb\base and so if two
different instances (for two different extensions) were created by the
same script they shared the same migrations list.

PHPBB3-12963
[ticket/12963] Load extensions migrations from /migration

PHPBB3-12963
[ticket/12963] Add unit tests

PHPBB3-12963