runtests: check ports on IPv6 address

Previously, checkportisavailable only checks ports on the IPv4 address. This
patch makes it check IPv6 as well. It'll be useful if "localhost" does not
have an IPv4 address, or its IPv4 address does not exist somehow.

tests/run-tests.py

@@ -114,13 +114,17 wifexited = getattr(os, "WIFEXITED", lam
 
 def checkportisavailable(port):
     """return true if a port seems free to bind on localhost"""
+    families = [getattr(socket, i, None)
+                for i in ('AF_INET', 'AF_INET6')
+                if getattr(socket, i, None) is not None]
+    for family in families:
     try:
-        s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+            s = socket.socket(family, socket.SOCK_STREAM)
         s.bind(('localhost', port))
         s.close()
         return True
     except socket.error as exc:
-        if not exc.errno == errno.EADDRINUSE:
+            if exc.errno not in (errno.EADDRINUSE, errno.EADDRNOTAVAIL):
             raise
         return False