Fixed a couple of issues with tests when running with no existing databa...

...ses:

- Temporary code in F1Context.CreateModel calls AddProperty which replaces an existing property with a new one, without updating the keys. That causes issues when we order properties in the DatabaseBuilder and rely on reference equality, resulting in duplicate columns. The fix is to move the code at the begining of the method so they don't get recreated.
- File not found exception about Northwind.sql because specified relative path goes up one directory too much.