This change adds a new parameter 'tree' to the get_build_test_report() method.
This new parameter is not added to the url but instead will be passed via request parameters.
Rationale for addding the change is to allow users to get only a subset of fields
because for huge test reports getting all fields may cause significant load on the Jenkins server.
Change-Id: I0c4c048bdb1c2a3ef75965695d8645cd280ce37e
8af7e92d47