Add Xcode 5 test action support.

This allows to have a dedicated build track for running unit/integration tests, making the CI setup clearer and less cumbersome.

I'm far from being a Jenkins plugin expert, so any feedback is more than welcome.
Specially, the way I'm passing the sim/os version to the XcodeTestDescriptor in doFill* is nasty, so suggested are more than welcome.

In a nutshell, this patch:
- Made XCodeBuild abstract, and subclassed by XcodeBuilder and XcodeTest
- Extracted XcodeDescriptor to an abstract class, for reuse by two newly created classes
- Added relevant Jelly config for XcodeTester