CA-146872: Ensure diskdatatest uses absolute path

Prior to this path, diskdatatests were using relative paths and run
under the assumption that tests would always be executed from XenCert's
root directory which caused a bunch of failures. This path should switch
the diskdatatests to use the absolute path.

Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
Reviewed-by: Mate Lakat <mate.lakat@citrix.com>

GitHub: closes #29 on xenserver/xencert