Added a hook on SLTest exposing failures that are encountered during a test run.

We'd like to display some more debugging information about a test failure when it happens, so we've added a hook to support this.
[`-isEqual:`'s docs](https://developer.apple.com/library/ios/documentation/cocoa/reference/foundation/Protocols/NSObject_Protocol/Reference/NSObject.html#//apple_ref/occ/intfm/NSObject/hash) say that you've got to override `-hash` too.