Run eslint tests with jest

Turns out eslint-tester works with jest (!)

We take advantage of this internally at FB, so we'll do it here too.

Fixes #4328

Gruntfile.js

@@ -114,7 +114,6 @@ module.exports = function(grunt) {
   ]);
   grunt.registerTask('build:test', [
     'delete-build-modules',
-    'test:eslint-rules',
     'jsx:test',
     'version-check',
     'populist:test',
@@ -133,8 +132,6 @@ module.exports = function(grunt) {
 
   grunt.registerTask('coverage:parse', require('./grunt/tasks/coverage-parse'));
 
-  grunt.registerTask('test:eslint-rules', require('./grunt/tasks/test-eslint-rules'));
-
   grunt.registerTask('test:webdriver:phantomjs', [
     'connect',
     'webdriver-phantomjs',

grunt/tasks/test-eslint-rules.js

@@ -1,20 +0,0 @@
-'use strict';
-
-var grunt = require('grunt');
-
-module.exports = function() {
-  var done = this.async();
-  grunt.util.spawn({
-    cmd: 'node_modules/eslint-tester/node_modules/mocha/bin/mocha',
-    args: ['eslint-rules/__tests__'],
-    opts: {stdio: 'inherit'}, // allows colors to passthrough
-  }, function(err, result, code) {
-    if (err) {
-      grunt.log.error('Custom linter rules are broken');
-    } else {
-      grunt.log.ok('Custom linter rules are okay');
-    }
-
-    done(code === 0);
-  });
-};

package.json

@@ -95,7 +95,8 @@
       "/.module-cache/"
     ],
     "testPathDirs": [
-      "<rootDir>/src"
+      "<rootDir>/src",
+      "<rootDir>/eslint-rules"
     ],
     "unmockedModulePathPatterns": [
       ""