docs: Link to unit tests from rule documentation

[btmills]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

[x] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

What changes did you make? (Give an overview)

In the Resources section at the bottom of each rule's documentation, we already link to the rule's implementation and the documentation's Markdown source. I've wished multiple times to have a handy link to the unit tests as well to see if a case is covered. This adds that link.

Is there anything you'd like reviewers to focus on?

For example, I ran npm run gensite locally, and here's the diff for the accessor-pairs documentation:

diff --git docs/rules/accessor-pairs.md docs/rules/accessor-pairs.md
index 8623782d..23de1f59 100644
--- docs/rules/accessor-pairs.md
+++ docs/rules/accessor-pairs.md
@@ -303,4 +303,5 @@ This rule was introduced in ESLint 0.22.0.
 ## Resources

 * [Rule source](https://github.com/eslint/eslint/tree/HEAD/lib/rules/accessor-pairs.js)
+* [Test source](https://github.com/eslint/eslint/tree/HEAD/tests/lib/rules/accessor-pairs.js)
 * [Documentation source](https://github.com/eslint/eslint/tree/HEAD/docs/rules/accessor-pairs.md)

[nzakas]

I was just thinking if this. 👍

[snitin315]

LGTM 👍🏻

[eslint-github-bot]

Hi @btmills!, thanks for the Pull Request

The first commit message isn't properly formatted. We ask that you update the message to match this format, as we use it to generate changelogs and automate releases.

• The commit message tag wasn't recognized. Did you mean "docs", "fix", or "feat"?
• There should be a space following the initial tag and colon, for example 'feat: Message'.
• The first letter of the tag should be in lowercase

Read more about contributing to ESLint here

[aladdin-add]

marked accepted, as no objections from the team. 😄