Removed the `Dbal` prefix from the class names and moved them to the `Doctrine\Tests\DBAL` namespace.

Add unit test to check boolean conversion with and without PDO boolean type

Add functional test to check null conversion with boolean type. This test required NULL to be allowed in the table column.

PG specific attribute PDO::PGSQL_ATTR_DISABLE_NATIVE_PREPARED_STATEMENT is now only set when it is still supported