bug-765564.t: fix path for file to rename

The script assumed it was chdir'ed into the mount point while it
was not. There also happened to be (conincidentally) a directory
of name 'a' in the workspace. The first the script was run, the
rename succeeded. All future tests are failing as the directory
already exists.

Change-Id: I92ad62e7893c03bb3f2af75c51d35bc35866dafe
Signed-off-by: Anand Avati <avati@redhat.com>
BUG: 765564
Reviewed-on: http://review.gluster.org/4442
Tested-by: Gluster Build System <jenkins@build.gluster.com>