How to log simpletest test results into files using drush test-run? I would like to save simpletest test results into a log file. What bash commands should I use ?

I tried this command (example) but nothing is saved into /var/tmp folder :

sudo -u apache drush test-run Comment --xml /var/tmp


I use Linux , Drupal 7.12, Drush 4.5
I don't use drush test-run, but Drush options are always in the form --xml=/var/tmp.

c.f. http://drupal.org/node/1416198

From drush help test-run

 --xml      Output verbose test results to a     
            specified directory using the JUnit  
            test reporting format. Useful for    
            integrating with Jenkins.  


Try:

sudo -u apache drush test-run Comment --all --xml=/var/tmp