Refactor spec helper method for clearer intent.
Rename spec helper for clarity.

Rename #generates_with_expected_string_output to #generates_with_expected_string_output_given_list, adding "_given_list" to explain the parameter.
Refactor EmployersPostedJobReportGenerator.

Refactor EmployersPostedJobReportGenerator#generate_from to use the new EmployerJobList class instead of filtering by poster manually.