StdlibUnittest: Add a default case to a switch over a resilient enum

I'm not completely satisfied with this fix because we get a compiler
warning when compiling without -enable-resilience. Perhaps we need
a policy change that a default case in a switch only generates a
warning if the enum is *explicitly* @_fixed_layout (or eventually,
@closed or however this attribute ends up being written).

Author: slavapestov

stdlib/private/StdlibUnittest/StdlibUnittest.swift.gyb

@@ -412,6 +412,8 @@ extension ProcessTerminationStatus {
       return false
     case .signal(let signal):
       return CInt(signal) == SIGILL || CInt(signal) == SIGTRAP
+    default:
+      fatalError("Bad ProcessTerminationStatus")
     }
   }
 }