wechat_secure_payment: mock portal.absolute_url to fix test

This tests had a test instance URL hardcoded and was working only on
this instance.
5 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.PerformanceTest-Master

00:32:36

passed SlapOS.Eggs.UnitTest-Master.Python2

00:17:30

passed SlapOS.Eggs.UnitTest-Master.Python3

00:19:15

passed SlapOS.Eggs.UnitTest-luke.Python2

00:17:23

passed SlapOS.Eggs.UnitTest-luke.Python3

00:19:05