test/racc/test_racc_command.rb: prevent a warning
test/racc/test_racc_command.rb: prevent a warning

http://rubyci.s3.amazonaws.com/ubuntu2004/ruby-master/log/20200929T003003Z.log.html.gz
```
warning: ambiguous first argument; put parentheses or a space even after `/' operator
```

tenderlove/racc,tenderlove/racc,tenderlove/racc,tenderlove/racc,tenderlove/racc