Add handling for XCTest bundles

An XCTest bundle is a CFBundle with a special product-type and bundle
extension. It gets loaded directly into the AppBundle or Framework
it should test. For more information about XCTest visit the Mac
Developer library at:

http://developer.apple.com/library/mac/documentation/DeveloperTools/Conceptual/testing_with_xcode/

To build a XCTest bundle, you need to create a CFBundle and set the
XCTEST property. Additionally you need to specify the application
or framework target to test within the XCTEST_HOST property. CMake
and Xcode will then setup the correct target dependencies.

add_library(CocoaExampleTest MODULE ...)
set_target_properties(CocoaExampleTest PROPERTIES
  XCTEST TRUE
  XCTEST_HOST CocoaExample)

Signed-off-by: Gregor Jasny <gjasny@googlemail.com>
Add XCTest Example

This test builds an trivial Cocoa App Bundle and an XCTest Bundle.
After project generation open it in Xcode and 'Test' the project.

Signed-off-by: Gregor Jasny <gjasny@googlemail.com>