Fix running extension unit tests that need the default logo.png.
Fix running extension unit tests that need the default logo.png.

A few of our unit test helpers try to use our in-tree `logo.png` as test
data. This is used for screenshots and file attachments. The path to
this is built using `settings.STATIC_ROOT`, and in-tree, this works
fine. However, if run in an extension's unit test suite,
`settings.STATIC_ROOT` won't point to the Review Board tree's `static`
directory, causing these to fail.

This change gets rid of the `STATIC_ROOT` usage and instead computes the
actual path to the bundled `static` directory. This will ensure that
tests will work without having to worry about `STATIC_ROOT`.

Testing Done:
Unit tests pass in Review Board and in extension trees that were failing.

Reviewed at https://reviews.reviewboard.org/r/11315/

reviewboard/reviewboard,chipx86/reviewboard,chipx86/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,chipx86/reviewboard