qemu-iotests: Treat custom TEST_DIR in 051

qemu-iotests: Treat custom TEST_DIR in 051

When custom TEST_DIR is specified the output includes it without leading
'/':

    $ TEST_DIR=/var/tmp ./check -file -qcow2 051
    ....
-drive0 (NODE_NAME): json:{"backing": {"driver": "qcow2", "file":
{"driver": "file", "filename": "TEST_DIR/t.qcow2"}}, "driver": "qcow2",
"file": {"driver": "file", "filename": SNAPSHOT_PATH}} (qcow2,
read-only)
+drive0 (NODE_NAME): json:{"backing": {"driver": "qcow2", "file":
{"driver": "file", "filename": "TEST_DIR/t.qcow2"}}, "driver": "qcow2",
"file": {"driver": "file", "filename": "TEST_DIR/vl.ziHfeP"}} (qcow2,
read-only)

Let's remove it from the sed regexp.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>