tests: Make test framework class useful for XGL testing

tests: Make test framework class useful for XGL testing

Add the class XglTestClass that is intended to be used by
XGL test programs and knows how to capture interesting
test events such as images.
This includes the ability to parse two command line options
--show-images: bring up a simple image window that will show any
images of note (those that an application says to record). Results
are saved so that the operator can scroll through the images
at the completion of the test.
--save-images: Output the image as a ppm file using the test name
as the file base name.