swift234

Implement an XCTest overlay with basic assertions

Adds an overlay for Xcode's XCTest testing framework.

It implements most of the familiar test assertion macros as equivalent
Swift functions. The assertion macros that aren't currently implemented
are only those that deal specifically with floating-point equality and
Objective-C exceptions. Additionally, the implemented assertions don't
currently handle Objective-C exceptions thrown out of some code called
during an assertion as test failures.

Swift SVN r15917