Don't prepend $DATA_DIR to volume path that already has it

Otherwise the test is testing the wrong file.

Change-Id: Ia7dcda8d4f3a1fe87e74f4605a7010b66c38fa14
diff --git a/lib/lvm b/lib/lvm
index ed24487..c183f09 100644
--- a/lib/lvm
+++ b/lib/lvm
@@ -86,7 +86,7 @@
     local backing_file=$DATA_DIR/$vg$BACKING_FILE_SUFFIX
     if ! sudo vgs $vg; then
         # Only create if the file doesn't already exists
-        [[ -f $DATA_DIR/$backing_file ]] || truncate -s $size $backing_file
+        [[ -f $backing_file ]] || truncate -s $size $backing_file
         local vg_dev=`sudo losetup -f --show $backing_file`
 
         # Only create volume group if it doesn't already exist