Removing these tests and simplifying templates.

The tests don't really focus on our code and mess about with DEBUG and RequestContext, which don't need testing.
We don't need a 'debug' template anymore.
Use 'mini' instead of request to make it obvious we aren't testing anything with RequestContext.
Template change - not necessary but a touch simpler. No line break in the rendered template either.
Refactor List and Iter tests to use the template tags as they are used in actual templates. I think this also makes the tests easier to understand.

Testing the Node instances seems disingenuous and it isn't how Django tests its own template tags.
Be explicit about template settings when testing. This doesn't "fix" anything but follows the steps Django uses to test its own templates.
Refactor `render_model` test.

- use the template tag as it is used in actual templates (same as the refactor to List and Iter)
- drop the unnecessarily complicated `make_str()` introduced in 5e242f6dc7f4eab3967af6536e43e3365e11b044. Now that I've worked with the entire file, we just don't need to swap out template names like this. Better to be explicit anyway.
- don't use render_model **object**. use a different variable name to be more obvious that this name is not important.
- replace the stub context managers with less confusing Fakes on the test where they are used
Additional tests for List and Iter
Add tests to check for too many/few parameters to List/Iter/Next/Remainder.
Add tests for filtering the arguments to all our template tags. 4 of the 6 tests break; we'll fix them in the next commit.
Filtering works on all arguments. Fixes the previous 4 broken tests.

Also use `obj` instead of `object`.