To allow execution of the tests with different configurations without having
to use the test_config.php file, environment variables of the form
PHPBB_TEST_<name> can now be used, e.g. PHPBB_TEST_DBMS to set the variables
otherwise expected in test_config.php
PHPBB3-9868
Tests are run with sqlite by default now anyway, so in the majority of cases
the error message explaining how to set up database test running will not be
displayed anyway. Database tests are now generally simply skipped if no
configuration can be found. The RUNNING_TESTS.txt file explains how to set
them up however, and more info is available on the wiki.
The get_database_config method was moved from test_case_helpers to
database_test_case because it has no general purpose.
PHPBB3-9868
