[ticket/13161] Avoid problems when running tests with different migrations

PHPBB3-13161
[ticket/13161] Check whether the schema exists, before deleting it

PHPBB3-13161
Merge pull request #3029 from nickvergessen/ticket/13161

Ticket/13161 PHP Warnings issued from phpbb database test case

* nickvergessen/ticket/13161:
  [ticket/13161] Check whether the schema exists, before deleting it
  [ticket/13161] Avoid problems when running tests with different migrations