Don't use wc -l for the umount check

We don't need the follow the pattern of checking wc -l against
/proc/mounts is 0, we care about any match, so we can just use
grep and it's exit code inside the if.

Change-Id: Idd0d78a613f8b43f169a320527c763d0bff74368

tests/dib_functions_test.sh

@@ -37,7 +37,7 @@ fi
 # umount devices
 unmount_dir $TMP_MOUNT_PATH
 
-if [ $(grep "$TMP_DIR" /proc/mounts | wc -l) -ne 0 ]; then
+if grep -q "$TMP_DIR" /proc/mounts; then
     echo "*** FAILED due to mounts being left behind"
     return 1
 else