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.

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