Change haystack INDEX_NAME when running tests

This uses the same technique sayit uses [1] for detecting when we're running
tests, however instead of switching to the SimpleEngine, we're
overriding the INDEX_NAME setting by appending '_test' to it.

Originally I did try using the SimpleEngine, but it caused errors when
running the tests. Doing it this way also means our tests run in an
environment that's closer to production.

[1]: https://github.com/mysociety/sayit/blob/master/spoke/settings/base.py#L231