Fix error in riece-log test

Fix error in riece-log test

The portable version of the recursive delete-directory doesn't seem to
work well with Emacs 23 or later, when a file name is encoded in a
multibyte encoding.  Utilize the RECURSIVE parameter as a workaround.
Reported by Hideki Yamane.
* test/test-riece-log.el (test-riece-log-delete-directory): Use
delete-directory with newly added RECURSIVE parameter, if possible.