Added XCTest target

I added an XCTest target and test (test_EXPFailWithTestCaseClassThatHasFailureMethod) to demonstrate that @tibr's XCTest support commit works correctly. I also updated all tests to subclass XCTestCase and use its assertion methods when run under the XCTest target.

The old test targets still run under Xcode 4, and all test targets run under Xcode 5 (assuming you configure your schemes appropriately).