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  
My personal opinion is that these tests should go in a different file. I'm not sure that future developers would think to look in `test-child-process-spawn-typeerror` for `execFile()` tests.