Test improvements.
Test improvements.

- use a context manager to test `_meta` model values. This replaces  `setUp()` and `tearDown()` with a more obvious mechanism.
- don't use testcase-wide models; setup the necessary models directly in the tests they are needed in.
- Foobar is a common model, so remove the duplicate code determining its template path
- in all cases the `expected` value is what it should be. Don't inconsistently coerce it into a list in the assertions.

armstrong/armstrong.core.arm_layout,armstrong/armstrong.core.arm_layout