Add examples how to run make test-cmd specific tests

Author: soltysh

build/root/Makefile

@@ -282,8 +282,12 @@ endif
 define TEST_CMD_HELP_INFO
 # Build and run cmdline tests.
 #
+# Args:
+#   WHAT: List of tests to run, check test/cmd/legacy-script.sh for names.
+#     For example, WHAT=deployment will run run_deployment_tests function.
 # Example:
 #   make test-cmd
+#   make test-cmd WHAT="deployment impersonation"
 endef
 .PHONY: test-cmd
 ifeq ($(PRINT_HELP),y)