#516 test_multimedia.py creating "test.ogg.ogg" in the users video directory

This fixes the issue where a file "test.ogg.ogg" was being created in the users video directory when running the test_multimedia.py.  This code now sets the videodir as a temporary directory before executing the test.   Setting the videodir before executing the test was implemented in aac07f1.

Link to issue: https://github.com/Freeseer/freeseer/issues/516