Tests: Fix RunCMake.CompileFeatures test on Xcode

In the NonValidTarget2 test case, mark the generated source file with
the GENERATED property explicitly since it is not marked automatically
as the output of any custom command.  Mark both the 0 and 1 variants
so it works no matter the outcome of the generator expression.
Merge topic 'AppleClang-5.1-features' into next

e7781c6b Tests: Fix RunCMake.CompileFeatures test on Xcode