Generalized Test Follow Up

[jmchilton]

A list of tasks to enhance the newer planemo capability to test Galaxy workflows, cwl tools (with Galaxy or cwltool), and cwl workflows (with cwltool). New testing was added in #454 and scoped out in #448.

• Document the <artifact>-test.yml format.
• Refactor much of Galaxy's testing comparison stuff out into galaxy-lib or a new testing package for reuse in planemo.
• Develop a test syntax to leverage above library from <artifact>-test.yml tests.
• Implement XUnit reports for newer testing stuff (html, markdown, json, and text already work).
• Extend CWL tool_init to build a test case.