Create inspection to report missing screenshot files in Terra.validates calls

[picimako]

Summary of scope

The scope is to implement an inspection for Terra.validates.screenshot(); and related calls, so that it would report when a "referenced" screenshot file doesn't exist.

Solution details

• The inspection would report the image missing if it doesn't exist for any context (locale, browser, viewport/formFactor).
• When the validated calls have a name parameter specified, the inspection would register the name parameter to the problems, otherwise it would the element representing the call's name (e.g. Terra.validates.screenshot) that would be registered.
• The calls to include:
  • Terra.validates.screenshot
  • Terra.validates.element
  • Terra.it.validatesScreenshot
  • Terra.it.validatesElement

Out of scope

• latest and diff versions of images won't be included in the validation, only the reference version of the image should be checked.

[picimako]

Done.