(maint) Refactor parsedfile specs to remove instance vars

The current test coverage for the ParsedFile provider uses an instance
variable referencing the class. This commit removes the instance
variable use and replaces them by setting `subject` to an anonymous
subclass of ParsedFile.
(maint) convert parsedfile helpers to let blocks

This commit replaces the use of instance variables with `let` blocks in
the parsedfile provider specs.