Always specify -sdk for xcrun.

Some tests using `import Foundation` erroneously pass because they cannot compile:

```
<unknown>:0: error: cannot load underlying module for 'CoreGraphics'
<unknown>:0: note: did you forget to set an SDK using -sdk or SDKROOT?
<unknown>:0: note: use "xcrun -sdk macosx swift" to select the default OS X SDK installed with Xcode
```

Always specifying an SDK for `xcrun` ensures the availability of necessary libraries.