overlay: rename OVERLAY_LOWER/UPPER/WORK_DIR

As preparation step for configuring test/scratch base fs
for overlay tests:

- Rename OVERLAY_LOWER/UPPER/WORK_DIR => OVL_LOWER/UPPER/WORK
  because we want to use OVL_ prefix for all base fs vars

- Prepend "ovl-" prefix to lower/upper/work path values to
  isolate the overlay test dirs when running on a base fs
  that is also used to run non overlay tests

- Make those vars values non-configurable, because lower/upper/work
  directory names are an internal test detail which should
  not concern the user and because we wish to simplify
  and document the overlay tests setup

- Add helper _filter_ovl_dirs and use it to filter output
  of test overlay/023 which matches the constant string "work"

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>

common/filter

@@ -424,5 +424,12 @@ _filter_lostfound()
 	sed -e '/^lost+found$/d'
 }
 
+_filter_ovl_dirs()
+{
+	sed -e "s,$OVL_LOWER,OVL_LOWER,g" \
+	    -e "s,$OVL_UPPER,OVL_UPPER,g" \
+	    -e "s,$OVL_WORK,OVL_WORK,g"
+}
+
 # make sure this script returns success
 /bin/true