added - edac_max_alt_length filter and edac_rule_img_alt_long tests #95

[SteveJonesDev]

This PR adds a filter to control the max alt length and unit tests for the image alt long rule.

• Added: edac_rule_img_alt_long filter that defaults to 300
• Added: unit test for the edac_rule_img_alt_long function
  • Test that function returns an empty array when all images have alt text within the allowed length
  • Test that function returns an array with errors when images have alt text longer than the allowed length
  • Test that the filter modifies the maximum allowed alt text length

Fixes #95