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
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
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.

You can run the example with:

  cmake -GXcode .
  xcodebuild -scheme CocoaExample test

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