LU-1075 tests: auster detect directory as script name

LU-1075 tests: auster detect directory as script name

Auster should use -f to detect the existing of
script name rather use -e which will treat directory
same as script.

Signed-off-by: Minh Diep <mdiep@whamcloud.com>
Signed-off-by: Yu Jian <yujian@whamcloud.com>
Change-Id: Ic05aeeb6cdda183dddfdc1bec2646546898922dd
Reviewed-on: http://review.whamcloud.com/4795
Tested-by: Hudson
Reviewed-by: Wei Liu <wei3.liu@intel.com>
Reviewed-by: Minh Diep <minh.diep@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>