Prevent prefix fail in caldav tests

tracim/test.ini

 radicale.server.port = 15232
 radicale.client.port = 15232
 radicale.server.filesystem.folder = /tmp/tracim_tests_radicale_fs
+radicale.client.base_url = http://localhost:15232
 
 [server:main]
 use = egg:gearbox#wsgiref