Skip collectstatic when running unit test suites

This has been driving me crazy :)  There is no reason why the unit test suite needs to run collectstatic: since view-level tests aren't actually rendering anything in the browser, none of the static asset links will be followed.

There were a few tests in `courseware_wiki` that depended on assets being available, because:
* Django templates were being used, which included...
* A `static` tag, which called...
* `pipeline.storage.PipelineCachedStorage`, which raised
* a `ValueError` if the resource was not found.

Except for the `static_replace` Django app, all other unit tests shouldn't need `PipelineCachedStorage`.

This PR updates the unit test settings, removes the `collectstatic` step in the rake test commands, and explicitly overrides settings to install `PipelineCachedStorage` for the few tests that actually need it.

https://edx-wiki.atlassian.net/browse/TE-280

@jzoldak