Add test case.

This tests a codepath in QR_next_tuple that had no coverage before.
Add test case for large objects.
Fix Windows compilation with libpq enabled, but without SSPI

GetUserNameEx function requires secur32.dll. It is now referenced in the
code, regardless of SSPI, so we need to link with secur32.dll.
Allow building test suite in a different place other than the source code is.
Change the test suite so that it works not only with unixODBC but also with iodbc.