Raise 'TestSkipped' (from the test_support) module rather than 'ImportError'

to signify a test that should be marked as 'skipped' rather than 'failed'.
Also 'document' it, in README.

Lib/test/test_fork1.py

@@ -9,11 +9,12 @@
 """
 
 import os, sys, time, thread
+from test_support import TestSkipped
 
 try:
     os.fork
 except AttributeError:
-    raise ImportError, "os.fork not defined -- skipping test_fork1"
+    raise TestSkipped, "os.fork not defined -- skipping test_fork1"
 
 LONGSLEEP = 2