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

1. Use the actual dsdriver distribution
2. Introduced `AbstractPlatform::getCharMaxLength()` since `CHAR` and `VARCHAR` limitations are different on DB2.

The SQLite and Oracle platforms produces a `ROUND()`'ed or `TRUNC()`'ated difference between the datetimes. However it should be the difference between dates.