Feature/fix child process type checking

https://github.com/joyent/node/pull/8392 introduced a test that doesn't fail when the code it is testing is deleted.

Also, behaviour of execFile and spawn have drifted apart again, unnecessarily, I think. This brings them into line again.

/to @cjihrig  
I'd prefer to rename it, so that when some future developer changes the signature of execFile they will think, "hey, I should change spawn, too!". OK?