Do you put unit tests in same project or another project?

Question

Do you put unit tests in the same project for convenience or do you put them in a separate assembly?

If you put them in a separate assembly like we do, we end up wit

Answer 1

If NUnit framework is used, there is an additional reason to put the tests in the same project. Consider the following example of the production code mixed with unit tests:

public static class Ext
{
     [TestCase(1.1, Result = 1)]
     [TestCase(0.9, Result = 1)]
     public static int ToRoundedInt(this double d)
     {
         return (int) Math.Round(d);
     }
}

The unit tests here serve as documentation and specification to the code being tested. I do not know how to achieve this effect of self-documenting, with the tests located in a separate project. The user of the function would have to search for the tests to see those test cases, which is unlikely.

Update: I know that such usage of TestCase attribute was not that the developers of NUnit intented, but why not?