There are many issues with the web service unit tests that I fixed in https://github.com/mouneyrac/local_wstesting/blob/master/simpletest/testwebservice.php

From memory the fixes are:

• REST server does not accept object parameters. I casted all object into array.
• some parameters name were incorrect. XML-RPC and SOAP were bypassing wrong name, only looking at the order. By REST was failing.
• Some functions were creating trash data and failed on second run.
• one of the new Moodle 2.2 web service function test is new: core_enrol_get_enrolled_users

Fixing in for 2.2 will be easy (it's about copying the file). Backporting in 2.1/2.0 is just a bit more work. After copying the file, take care about putting back the old moodle_enrol_get_enrolled_users instead core_enrol_get_enrolled_users, and also remove the possibility to test REST server (REST-JSON server only available in 2.2).