Split up GPML test files into individual test cases #25

My pull request #24 adds test cases. When I originally created these, each file contained multiple test cases. But having multiple test cases per file is hard to maintain, because when we add a new test case to a file, we invalidate all the existing test cases in that file.

To make things more maintainable, we need to split up these files into individual test cases:

• curves.gpml.xml
• elbows.gpml.xml
• double-edge.gpml
• data-nodes.gpml.xml
• fill-and-stroke.gpml.xml
• graphical-lines.gpml.xml
• groups.gpml.xml
• interactions.gpml.xml
• labels.gpml.xml
• publication-xrefs.gpml.xml
• shapes.gpml.xml
• size-and-proportion.gpml.xml
• text-and-font.gpml.xml
• z-index.gpml.xml

I did the ones for curves and elbows. Could anyone help with the others?