Automatically set up Mockery for unit tests

- Use provided PhpUnit listener to enforce verification of expectations.
- Include Mockery's trait to auto-close Mockery after each test.

tests/phpunit.unit.xml

@@ -19,4 +19,7 @@
             <directory suffix=".php">./src/</directory>
         </whitelist>
     </filter>
+    <listeners>
+        <listener class="\Mockery\Adapter\Phpunit\TestListener"></listener>
+    </listeners>
 </phpunit>