refactor: use explicit mocha imports (#497)

Author: JustinBeckwith

packages/google-cloud-dialogflow/system-test/.eslintrc.yml

@@ -1,5 +1,3 @@
 ---
-env:
-  mocha: true
 rules:
   no-console: off

packages/google-cloud-dialogflow/test/.eslintrc.yml

@@ -15,6 +15,7 @@
 'use strict';
 
 const assert = require('assert');
+const {describe, it} = require('mocha');
 const {PassThrough} = require('stream');
 
 const dialogflowModule = require('../src');

packages/google-cloud-dialogflow/test/gapic-v2.js

@@ -15,6 +15,7 @@
 'use strict';
 
 const assert = require('assert');
+const {describe, it} = require('mocha');
 const {PassThrough} = require('stream');
 
 const dialogflowModule = require('../src');