A11y include input attributes

[seanjh]

why

Resolves #224 by including additional AXProperty values in the simplified AX tree

what changed

• Added a new unit test suite for the a11y utilities
• Only include truthy AX property values in the formatted output (e.g., selected=true, but not selected=false)
• Expose additional AXProperty values on their corresponding node in the simplified AX tree
  • selected (ref)
  • checked (ref)
  • valuemin (ref)
  • valuemax (ref)
  • valuetext (ref)
  • valuenow (ref)

test plan

• Add simple HTML files to test/fixtures/html_pages
• Dump the corresponding Accessibility.getFullAXTree contents for these pages to test/fixtures/ax_trees
• Validate the behavior of a11y functions given against static AX trees