run-tests: resurrect the wifexited polyfill (backout 6ab5a1c9ea3c)

Python 2.7.3 on Windows doesn't have os.WIFEXITED, and the test output looked
like this before I interrupted it.

    $ ./run-tests.py --local -j2 -t700
    EEEEEEEEEEEEEEEEEEEEEE

tests/run-tests.py

@@ -103,6 +103,9 b' else:'
 
     _strpath = _bytespath
 
+# For Windows support
+wifexited = getattr(os, "WIFEXITED", lambda x: False)
+
 def checkportisavailable(port):
     """return true if a port seems free to bind on localhost"""
     try:
@@ -804,7 +807,7 b' class Test(unittest.TestCase):'
             raise
 
         ret = proc.wait()
-        if os.WIFEXITED(ret):
+        if wifexited(ret):
             ret = os.WEXITSTATUS(ret)
 
         if proc.timeout:
@@ -905,7 +908,7 b' class TTest(Test):'
                       self._testtmp, 0, self._getenv())
         stdout, stderr = proc.communicate()
         ret = proc.wait()
-        if os.WIFEXITED(ret):
+        if wifexited(ret):
             ret = os.WEXITSTATUS(ret)
         if ret == 2:
             print(stdout)