New: RuleTester supports processor

designs/2019-rule-tester-processors/README.md

@@ -0,0 +1,70 @@
+-   Start Date: 2019-07-16
+-   RFC PR: (leave this empty, to be filled in later)
+-   Authors: Toru Nagashima ([@mysticatea](https://github.com/mysticatea))
+
+# RuleTester supports processor
+
+## Summary
+
+This RFC makes `RuleTester` class supporting `processor` option.
+
+## Motivation
+
+Currently, we cannot test rules with processors. This is inconvenient for plugin rules which depend on processors. For example, [vue/comment-directive](https://github.com/vuejs/eslint-plugin-vue/blob/6751ff47b4ecd722bc2e2436ce6b34a510f92b07/tests/lib/rules/comment-directive.js) rule could not use `RuleTester` to test.
+
+## Detailed Design
+
+To add `processor` option and `processorOptions` (see #29) to test cases.
+
+```js
+const { RuleTester } = require("eslint")
+const rule = require("../../../lib/rules/example-rule")
+const exampleProcessor = require("../../../lib/processors/example-processor")
+
+const tester = new RuleTester()
+
+tester.run("example-rule", rule, {
+    valid: [
+        {
+            code: `
+                <script>
+                    console.log("Hello")
+                </script>
+            `,
+            processor: exampleProcessor,
+            processorOptions: {},
+        },
+    ],
+    invalid: [],
+})
+```
+
+### § `processor` option
+
+This is a definition object of a processor that the "[Processors in Plugins](https://eslint.org/docs/developer-guide/working-with-plugins#processors-in-plugins)" section describes.
+
+If this option was given, the tester applies the given processor to test code.
+
+If the given processor didn't has `supportsAutofix:true`, the tester doesn't do autofix. Then if the test case had `output` option (except `null`) then the test case will fail.
+
+### § `processorOptions` option
+
+RFC #29 defines the `processorOptions`.
+
+If this option was given along with `processor` option, it will be given to the processor.
+
+## Documentation
+
+The [RuleTester](https://eslint.org/docs/developer-guide/nodejs-api#ruletester) section should describe the new `processor` and `processorOptions` properties.
+
+## Drawbacks
+
+It's a pretty rare case of a rule depends on the processor's behavior. I'm not sure if this feature is needed.
+
+## Backwards Compatibility Analysis
+
+There are no concerns about breaking changes.
+
+## Related Discussions
+
+-   https://github.com/eslint/rfcs/pull/25#issuecomment-499877621