test: point drain-timeout assertion at _CUA_INSTALLER_DRAIN_GRACE

The #87196/#87720 conflict resolution kept the bounded-drain helper and
its constant; the windows_only kill-tree test still asserted the dropped
_CUA_INSTALLER_REAP_TIMEOUT name. Same 2-communicate contract, surviving
constant.

tests/hermes_cli/test_install_cua_driver.py

@@ -1079,7 +1079,7 @@ class TestInstallerTimeoutKillsProcessGroup:
         assert fake_proc.communicate.call_count == 2
         assert (
             fake_proc.communicate.call_args_list[1].kwargs["timeout"]
-            == tools_config._CUA_INSTALLER_REAP_TIMEOUT
+            == tools_config._CUA_INSTALLER_DRAIN_GRACE
         )
 
     @pytest.mark.windows_only