feat(@embark/test-runner): introduce artifacts.require API

This commit adds a convenience API `artifacts.require(name)` that aims to make
requiring artifacts a little bit more straight forward.

Usage:

```
const SimpleStorage = artifacts.require('SimpleStorage');
const EmbarkJS = artifacts.require('EmbarkJS');
```

Author: 0x-r4bbit

dapps/tests/app/test/simple_storage_spec.js

@@ -1,7 +1,7 @@
 /*global contract, config, it, assert, web3*/
-const SimpleStorage = require('Embark/contracts/SimpleStorage');
+const SimpleStorage = artifacts.require('SimpleStorage');
 let accounts;
-const {Utils} = require('Embark/EmbarkJS');
+const {Utils} = artifacts.require('EmbarkJS');
 
 config({
   contracts: {

packages/plugins/mocha-tests/src/lib/index.js

@@ -222,6 +222,15 @@ class MochaTestRunner {
                 global.config = config;
               });
 
+              global.artifacts = {
+                require: (name) => {
+                  if (name === 'EmbarkJS') {
+                    return EmbarkJS;
+                  }
+                  return require(`Embark/contracts/${name}`);
+                }
+              }
+
               mocha.suite.timeout(TEST_TIMEOUT);
               mocha.addFile(file);