Fix unit test failure

* Fixed enforcer plugin failure due to TestNG mismatch
versions.
* Bumping TestNG dependency to 6.8.8 triggered JUnit
assert class not found. So fixed all unit tests to use
TestNG asserts.
* Fixed the build failure for the module ios-grid
due to missing testng xml file.
@lukeis and @freynaud 

Here's why that was added up as a dependency.

Currently there is a Test class named `org.uiautomation.ios.grid.IOSMutableProxyTest` which when executed throws the below mentioned exception if this `test` scope dependency is NOT added.

```
Running TestSuite
FAILED CONFIGURATION: @BeforeClass setup
java.lang.NoClassDefFoundError: org/seleniumhq/jetty7/util/thread/ThreadPool
	at org.uiautomation.ios.grid.IOSMutableProxyTest.setup(IOSMutableProxyTest.java:72)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
```
The line of code that is triggering the above exception is `hub = new org.openqa.grid.web.Hub(c);`
I wanted this test class to be executed and pass as well (This test class will now run because I have added a testng.xml file (Which the grid/pom.xml is referring and never existed until my pull request). The testng.xml that I added attempts to run all tests under `org.uiautomation.ios.grid.*`

Hope that clarifies. Also please note that the dependency I have added is ONLY a `test` scope to facilitate the above mentioned unit test execution.