test/system: Use the same checks for the toolbox(1) manual

... for both 'toolbox help' and 'toolbox --help'.

https://github.com/containers/toolbox/pull/1386

test/system/002-help.bats

@@ -47,6 +47,7 @@ setup() {
   assert_success
   assert_line --index 0 --partial "toolbox(1)"
   assert_line --index 0 --partial "General Commands Manual"
+  assert_line --index 3 --partial "toolbox - Tool for containerized command line environments on Linux"
   assert [ ${#lines[@]} -gt 4 ]
   assert [ ${#stderr_lines[@]} -eq 0 ]
 }