Another way of colocating test and code under test,
is to add a test method to a functional class.

    class ClassThatReallyGetsUsed {
    public:
       ...
    public:
       void test() {
          ...
       }
    };

Colocation is good, up until the point where 
the amount of test code starts distracting you from the actual code.

I nevertheless like using this pattern in a more restricted way:
even when I have created a separate test class,
my convention is to give it a test() method.
This allows separate test classes and ClassesWithTestMethod
to be managed using the same TestCollector.