Fix SF #1373161, r41552 broke test_file on OS X

You apparently can seek(0) on sys.stdin on OS X.
But you can't go backwards, so seek(-1).
diff --git a/Lib/test/test_file.py b/Lib/test/test_file.py
index 2869ce7..0803490 100644
--- a/Lib/test/test_file.py
+++ b/Lib/test/test_file.py
@@ -101,7 +101,7 @@
 f.close()
 
 try:
-    sys.stdin.seek(0)
+    sys.stdin.seek(-1)
 except IOError:
     pass
 else: