End to end testing for `RDoc#generate`

[st0012]

Currently no tests cover RDoc#generate which means we're not testing:

• The file generation logic
• The output message of doc generation

To test it right, we need to make sure we generate the documentation in a isolated place (e.g. a temp folder) and the files must be cleaned up after each test run.

[BurdetteLamar]

"End-to-end" means (?):

• Front end (input) is:
  • .rb files with embedded doc.
  • .c files with embedded doc.
  • .md files.
  • .rdoc files.
• Back end (output) is:
  • HTML files (darkfish).
  • RI files.