Fix system tests for Go 1.5.

Some error strings in Go 1.5 have changed. Until we have something
better for that, use string replace.

system/t04_mirror/create.py

@@ -231,6 +231,7 @@ class CreateMirror20Test(BaseTest):
     create mirror: using failing HTTP_PROXY
     """
     fixtureGpg = True
+    outputMatchPrepare = lambda _, s: s.replace('getsockopt: ', '')
 
     runCmd = "aptly -architectures='i386' mirror create -keyring=aptlytest.gpg -with-sources mirror20 http://security.debian.org/ wheezy/updates main"
     environmentOverride = {"HTTP_PROXY": "127.0.0.1:3137"}