Refactor the ParsedFile provider unit test

The initial implementation of the unit tests for a simple provider
inheriting ParsedFile was imperfect and full of repitition. The
restructuring makes the code reflect the semantics a lot better.

Based on suggestions from Adrien Thebo.
(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.
Merge branch 'pull-1494'

This closes GH-1494