add support for phpunit listeners configured via phpunit config

To add a listener to the phpunit.xml config like below. You can now add listeners to your phpunit-task
build.xml

```xml
     <!-- .... -->
    <target name="test" depends="prepare">
        <phpunit haltonfailure="true" haltonerror="true" printsummary="true" configuration="${project.basedir}/phpunit.xml.dist">
     <!-- .... -->
```


phpunit.xml 
```xml
    <!-- .... -->
    <listeners>
        <listener class="\Mockery\Adapter\Phpunit\TestListener"></listener>
    </listeners>
   <!-- .... -->
```