With the integration of testing base classes written by Roni
Choudhury, we needed to refactor the existing web-based testing
scripts.  The introduction of these new base classes allows us
to support multiple baseline images and testing in different
browsers seamlessly.

Now, new test scripts should extend one of the three new testing
base classes: WebTest, BrowserBasedWebTest, or
ImageComparatorWebTest, and override some or all of the testing
phase methods implemented in those classes.  New tests can be
given any class name, and the class will be instantiated
dynamically at run-time by the newly refactored vtk.web.testing
module.

Also changed was the FindSeleniumDrivers cmake script, so that
at least the firefox webdriver extension can be found.  More
changes were required in the ParaViewTestingMacros cmake file,
where I fixed a bug that kept any but the first browsers tests
from being registered, and where I added a new argument to the
addit...