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

tests/test-setdiscovery.t

@@ -403,6 +403,7 b' fixed in 86c35b7ae300:'
   searching for changes
   101 102 103 104 105 106 107 108 109 110  (no-eol)
   $ hg -R r1 --config extensions.blackbox= blackbox
+  * @5d0b986a083e0d91f116de4691e2aaa54d5bbec0 (*)> serve --cmdserver chgunix * (glob) (chg !)
   * @5d0b986a083e0d91f116de4691e2aaa54d5bbec0 (*)> outgoing r2 *-T{rev} * (glob)
   * @5d0b986a083e0d91f116de4691e2aaa54d5bbec0 (*)> found 101 common and 1 unknown server heads, 2 roundtrips in *.????s (glob)
   * @5d0b986a083e0d91f116de4691e2aaa54d5bbec0 (*)> -R r1 outgoing r2 *-T{rev} * --config *extensions.blackbox=* exited 0 after *.?? seconds (glob)