fix issue where error message wasn't being shown for device tests.

When doing run-tests on the iphoneos SDK, the user wasn't being the
shown the right error message.

OCUnitIOSDeviceTestRunner didn't have the right signature for the
runTests method.
make testSDKMustBeValid pass.

It wasn't using faked xcodebuild output when calling
GetAvailableSDKsAndAliases(), and so it would sometimes fail if you
didn't have the right SDKs installed locally.