Set context class loader when creating ORC writer

The ORC memory manager ends up loading VersionInfo which loads classes
from the context class loader. The tests now set an empty class loader
to verify that it uses the correct class loader.
Doing this will cause all class loading from this class loader to fail including classes from `java.*` which might be lazy loaded.  You might want to create a classloader with a `null` parent and then override `findClass` instead.