Merge GPML test cases into this repo

[ariutta]

I notice we have multiple GPML test suites in different locations:

• https://github.com/PathVisio/libGPML/tree/master/testData
• https://github.com/PathVisio/pathvisio/tree/master/testData
• https://github.com/PathVisio/pathvisio/tree/master/example-data
• https://github.com/wikipathways/gpml2pvjson-js/tree/master/test/input
• https://github.com/wikipathways/pvjs/tree/master/test/input-data

It would be good to merge these all into a single suite of test cases so we can easily use them to test any of our projects that deal with GPML, such as PathVisio, pvjs and the WikiPathways importer for Cytoscape. I got this started by moving my pvjs test cases into the test directory.