[ticket/11386] Use finder to find migration files

PHPBB3-11386
[ticket/11386] Update tests with new constructors for ext.manager/migrator

PHPBB3-11386
[ticket/11386] Forgot to get the migration classes

PHPBB3-11386
[ticket/11386] Remove tests that check if finder cache is working

These don't seem necessary and are much more complicated to get
working now with the changes in this PR

PHPBB3-11386
[ticket/11386] Fix circular reference error & serialize error

PHPBB3-11386
Merge branch 'develop' of git://github.com/phpbb/phpbb3 into ticket/11386

Conflicts:
	phpBB/includes/extension/finder.php
[ticket/11386] Fix failing tests from constructor changes

PHPBB3-11386
Merge remote-tracking branch 'EXreaction/ticket/11386' into develop

# By Nathaniel Guse
# Via Nathaniel Guse
* EXreaction/ticket/11386:
  [ticket/11386] Fix failing tests from constructor changes
  [ticket/11386] Fix circular reference error & serialize error
  [ticket/11386] Remove tests that check if finder cache is working
  [ticket/11386] Forgot to get the migration classes
  [ticket/11386] Update tests with new constructors for ext.manager/migrator
  [ticket/11386] Use finder to find migration files