Fix tests part 2

More test-related refactoring, fixing, and efficiency increases
* Moving utils/testing to shared/testing (nothing generic about our shared test code)
* Setting a static public & private key pair for every test case, so it's never generated on the fly (efficiency), and making sure tests use this base KALiteTestCase
* Using a persistent browser within a class, to avoid waiting for the browser to load constantly.

And the one (minor) actual code change (that is more inline with release-0.10.0):
* Do not cache the result of Device.get_own_device() in securesync.engine.models

I will merge this straightaway, as these tests are needed for release-idoc, and we're far behind in *actual* code pull requests.  Feel free to review at a later date, if there's time / interest!