Will/fix rake test

* Removes `--no-run-if-empty` option from `xargs`, which is not allowed on Mac OS X.

* Adds check to make sure the command does not run if the `reports` directory does not exist.

* Ignores errors from the clean command, which can occur on Ubuntu if `rm` is not passed any files.

@jzoldak can you please review?  This needs to be merged ASAP to fix the rake test command.