[unittest] Disambiguate references to errno

In Xcode 16 SDKs there seem to be two `errno` visible for some files,
one in the `Darwin` module, and one in the `_errno` module (which seems
new for this Xcode version).

Disambiguate the references by prepending `Darwin`, which should be the
one that was being used before Xcode 16 SDKs.

Author: drodriguez

stdlib/private/StdlibUnittest/StdlibUnittest.swift

@@ -1106,7 +1106,7 @@ class _ParentProcess {
       var ret: CInt
       repeat {
         ret = _stdlib_select(&readfds, &writefds, &errorfds, nil)
-      } while ret == -1  &&  errno == EINTR
+      } while ret == -1  &&  Darwin.errno == EINTR
       if ret <= 0 {
         fatalError("select() returned an error")
       }

stdlib/private/SwiftReflectionTest/SwiftReflectionTest.swift

@@ -303,7 +303,7 @@ internal func sendReflectionInfos() {
 
 internal func printErrnoAndExit() {
   debugLog("BEGIN \(#function)"); defer { debugLog("END \(#function)") }
-  let errorCString = strerror(errno)!
+  let errorCString = strerror(Darwin.errno)!
   let message = String(validatingCString: errorCString)! + "\n"
   let bytes = Array(message.utf8)
   fwrite(bytes, 1, bytes.count, stderr)