Improve test coverage, close a few loopholes.

[ianjosephwilson]

• Major changes
  • Improve and refactor tests for parser.py and processor.py
  • Non-literal aria and data attributes must now be None or an instance of dict
    • we can relax this later if it is a problem
  • Do not allow aria or data to be used as a TTemplatedAttribute
    • t'<div aria="{start}--{end}">' does not have a meaning right now
  • Allow None as an ignored value for spread attributes
    • should enable def my_comp(div_attrs=None): .... t'<div {div_attrs}></div>'
  • Adjacent TText tnodes are now "combined" during parsing

[davepeck]

Merged; thanks!