Fix wrong userSortField in userManager test config deployed by the WebEngineFeature

In /nuxeo-platform-test/src/main/resources/test-usermanagerimpl/userservice-config.xml, instead of:

<userSortField>name</userSortField>

which is an invalid field name for the user directory, we should have:

<userSortField>username</userSortField>

Otherwise the UserManager#searchUsers(String pattern) method will fail in tests using the WebEngineFeature that deploys this test config.