Add cases .ObjCRuntime and .NativeRuntime to StdlibUnittest's TestPredicate

[glessard]

This predicate case evaluates to false #if _runtime(_ObjC), true otherwise.

[gribozavr]

I would phrase it avoiding the negation: case ObjCRuntime, case NativeRuntime. What do you think?

[glessard]

I like that. Are you thinking two different cases, then?

[glessard]

Replaced the .noObjC case by paired .ObjCRuntime and .NativeRuntime cases

@gribozavr While we're at it, I was wondering whether cases .Always and .Never should also exist. There are a lot of uses of .Custom({true}, "") around.

[jrose-apple]

I'd personally rather name these "Apple" and "NonApple", rather than using the presence of the runtime as a stand-in. (And if we start testing swift-corelibs-foundation on OS X as "non-objc-runtime", we'll need that distinction!)

[glessard]

Seems to me that referring to the runtime is more precise; and your swift-corelibs-foundation aside appears to undermine the "Apple" and "NonApple" nomenclature. Surely I am misunderstanding?

[gribozavr]

Let's deal with swift-corelibs-foundation distinction later. I'm not sure it will even come up, because this repository can't depend on corelibs-foundation -- that would be circular.

For now, for compile-time requirements and for lit REQUIRES tags we have been using the runtime distinction, and I would prefer to keep it the same everywhere, at least to eliminate the subtle differences.

I agree about .Always and .Never, could you submit that as a separate pull request?

[gribozavr]

@swift-ci Please test