fix: rule 017 (long code) doesn't work as expected

[dwilding]

In #149 we want to split rule 017 into two rules. This PR does some preliminary work:

• Adds test cases for the existing behavior of rule 017. The tests identified some issues with how the rule works:
  • Acceptable/non-acceptable lengths don't match with "40 or fewer" in the error message
  • The rule doesn't account for blank lines in code introduced with :: (rst)
  • The rule doesn't (correctly?) account for format specifiers like .. code:: text (rst)
• TODO Fixes the rule.

I'll split the rule in a follow-up PR.