sprout test helpers

[Kyle MacDonald]

as a template creator, i want sprout test helpers to allow me easier assertions. to help think of what these should be, here are a few customary tests you'll be looking to assert with every template test suite

• add and init a template w/ overrides
• check if a file exists, and has certain content
• check if a file doesn't exists
• verify the contents of a template
• ensure override validation worked
• ensure override filter worked
• remove a template from sprout list
• remove the template generated output

[Wil (₩) Moore III]

Also, for testing purposes, I do believe the API should have sync alternatives. A solid pattern is that if no callback is provided, the function goes sync.