test: fix IPv6 support detection

`test_ipv6_local()` could have returned false whereas subsequently
bitcoind successfully binds to `::`.

This is because the previously used `s.connect(('::1', 0))` can throw
an exception like `[Errno 99] Cannot assign requested address` on hosts
where IPv6 is available and bitcoind readily binds to `::`.

Author: vasild

test/functional/test_framework/netutil.py

@@ -146,12 +146,15 @@ def test_ipv6_local():
     Check for (local) IPv6 support.
     '''
     import socket
-    # By using SOCK_DGRAM this will not actually make a connection, but it will
-    # fail if there is no route to IPv6 localhost.
-    have_ipv6 = True
-    try:
-        s = socket.socket(socket.AF_INET6, socket.SOCK_DGRAM)
-        s.connect(('::1', 0))
-    except socket.error:
-        have_ipv6 = False
-    return have_ipv6
+    if socket.has_ipv6:
+        s = None
+        try:
+            s = socket.socket(socket.AF_INET6, socket.SOCK_STREAM)
+            s.bind(('::', 0))
+            return True
+        except OSError:
+            pass
+        finally:
+            if s:
+                s.close()
+    return False