Integrated an object-oriented, phased testing approach for web tests.

Added and refactored some base classes written by Roni Choudhury
to the vtk.web.testing module.  These classes introduce a phased
testing approach where subclasses have an opportunity to override
the 5 phases of testing: 1) initialization, 2) setup, 3) capture,
4) postprocess, and 5) cleanup.

Now the test scripts are written as subclasses of one of the three
new testing base classes and override some or all of the testing
phases.  The scripts are dynamically loaded as modules, and then
inspection is used to find the class to instantiate at run time.

The integration of these test classes also provides the ability
to use vtk.vtkTesting to achieve support for multiple baseline
images.  An additional benefit of the testing classes is easy
support for multiple browsers.

Change-Id: I4de223e9b7fce4ba1b59ba21ed46c6bbc1e9d5c5
Merge topic 'pvweb-integrate-test-packages-alternate' into master

16145dfc Integrated an object-oriented, phased testing approach for web tests.