Added the parameter reportSuffix to UIAutomator

I added the parameter reportSuffix to the UIAutomator mojo. This makes it possible for the user to run two consecutive uiautomator tests without losing the first result xml. 

I did not want to have the parameter to set the name of the report completely (something like surefire's outputName). This is because if you run mvn surefire-report:report it will try to find all documents in target/surefire-reports/ that are named TEST-*.xml. The suffix parameter will therefore help the user not making any mistakes. 

I also changed the unit tests of UIAutomator to reflect the new parameter.