test-setdiscovery: make test compatible with chg The test checks the output of the blackbox extension which will contain logs corresponding to chg in case chg is running. Therefore, this commit modifies the test to take chg into consideration while working with the blackbox extension. Test Plan: Ran the test 'test-setdiscovery.t' with and without the '--chg' option. Differential Revision: https://phab.mercurial-scm.org/D926