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/README

@@ -12,7 +12,8 @@ functionality.  The mechanics of how the test system operates are fairly
 straightforward.  When a test case is run, the output is compared with the
 expected output that is stored in .../Lib/test/output.  If the test runs to
 completion and the actual and expected outputs match, the test succeeds, if
-not, it fails.  If an ImportError is raised, the test is not run.
+not, it fails.  If an ImportError or test_support.TestSkipped error is
+raised, the test is not run.
 
 You will be writing unit tests (isolated tests of functions and objects
 defined by the module) using white box techniques.  Unlike black box

Lib/test/test_binhex.py

@@ -6,7 +6,7 @@
 """
 import binhex
 import tempfile
-from test_support import verbose
+from test_support import verbose, TestSkipped
 
 def test():
 
@@ -15,7 +15,7 @@ def test():
         fname2 = tempfile.mktemp()
         f = open(fname1, 'w')
     except:
-        raise ImportError, "Cannot test binhex without a temp file"
+        raise TestSkipped, "Cannot test binhex without a temp file"
 
     start = 'Jack is my hero'
     f.write(start)

Lib/test/test_dl.py

@@ -4,7 +4,7 @@
 """
 
 import dl
-from test_support import verbose
+from test_support import verbose,TestSkipped
 
 sharedlibs = [
     ('/usr/lib/libc.so', 'getpid'),
@@ -29,4 +29,4 @@ for s, func in sharedlibs:
             print 'worked!'
         break
 else:
-    raise ImportError, 'Could not open any shared libraries'
+    raise TestSkipped, 'Could not open any shared libraries'

Lib/test/test_fork1.py

@@ -9,11 +9,12 @@ active threads survive in the child after a fork(); this is an error.
 """
 
 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
 

Lib/test/test_gl.py

@@ -3,7 +3,7 @@
     taken mostly from the documentation.
     Roger E. Masse
 """
-from test_support import verbose
+from test_support import verbose, TestSkipped
 import gl, GL, time
 
 glattrs = ['RGBcolor', 'RGBcursor', 'RGBmode', 'RGBrange', 'RGBwritemask',
@@ -87,8 +87,7 @@ def main():
     try:
         display = os.environ['DISPLAY']
     except:
-        # Raise ImportError because regrtest.py handles it specially.
-        raise ImportError, "No $DISPLAY -- skipping gl test"
+        raise TestSkipped, "No $DISPLAY -- skipping gl test"
 
     # touch all the attributes of gl without doing anything
     if verbose:

Lib/test/test_nis.py

-from test_support import verbose, TestFailed
+from test_support import verbose, TestFailed, TestSkipped
 import nis
 
 print 'nis.maps()'
@@ -9,7 +9,7 @@ except nis.error, msg:
     if verbose:
         raise TestFailed, msg
     # only do this if running under the regression suite
-    raise ImportError, msg
+    raise TestSkipped, msg
 
 done = 0
 for nismap in maps:

Lib/test/test_openpty.py

 # Test to see if openpty works. (But don't worry if it isn't available.)
 
 import os
-from test_support import verbose, TestFailed
+from test_support import verbose, TestFailed, TestSkipped
 
 try:
     if verbose:
@@ -10,7 +10,7 @@ try:
     if verbose:
         print "(master, slave) = (%d, %d)"%(master, slave)
 except AttributeError:
-    raise ImportError, "No openpty() available."
+    raise TestSkipped, "No openpty() available."
 
 if not os.isatty(master):
     raise TestFailed, "Master-end of pty is not a terminal."

Lib/test/test_pty.py

 import pty, os, sys, string
-from test_support import verbose, TestFailed
+from test_support import verbose, TestFailed, TestSkipped
 
 TEST_STRING_1 = "I wish to buy a fish license."
 TEST_STRING_2 = "For my pet fish, Eric."
@@ -25,7 +25,7 @@ try:
     debug("Got slave_fd '%d'"%slave_fd)
 except OSError:
     # " An optional feature could not be imported " ... ?
-    raise ImportError, "Pseudo-terminals (seemingly) not functional."
+    raise TestSkipped, "Pseudo-terminals (seemingly) not functional."
 
 if not os.isatty(master_fd):
     raise TestFailed, "master_fd is not a tty"

Lib/test/test_signal.py

 # Test the signal module
-from test_support import verbose
+from test_support import verbose, TestSkipped
 import signal
 import os
 import sys
 
 if sys.platform[:3] in ('win', 'os2'):
-    raise ImportError, "Can't test signal on %s" % sys.platform[:3]
+    raise TestSkipped, "Can't test signal on %s" % sys.platform[:3]
 
 if verbose:
 	x = '-x'

Lib/test/test_string.py

-from test_support import verbose
+from test_support import verbose, TestSkipped
 import string_tests
 import string, sys
 
@@ -6,7 +6,7 @@ import string, sys
 try:
     ''.join
 except AttributeError:
-    raise ImportError
+    raise TestSkipped
 
 def test(name, input, output, *args):
     if verbose: