test: fix shellcheck warnings in test_btrfs_clone.sh

- Use grep -q instead of comparing output with [ -z/n .. ] (SC2143)
- Quote $(id -u) to prevent word splitting (SC2046)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: andy5995

test/test_btrfs_clone.sh

@@ -19,9 +19,9 @@ if [ ! -d "$BTRFS_MOUNTPOINT" ]; then
   sudo mkdir "$BTRFS_MOUNTPOINT"
 fi
 
-if [ -z "$(mount | grep rmw-btrfs)" ]; then
+if ! mount | grep -q rmw-btrfs; then
   sudo mount -o loop "$BTRFS_IMAGE" "$BTRFS_MOUNTPOINT"
-  sudo chown $(id -u) -R "$BTRFS_MOUNTPOINT"
+  sudo chown "$(id -u)" -R "$BTRFS_MOUNTPOINT"
 fi
 
 cd "$BTRFS_MOUNTPOINT"
@@ -109,7 +109,7 @@ test -f foo
 
 cd
 
-if [ -n "$(mount | grep rmw-btrfs)" ]; then
+if mount | grep -q rmw-btrfs; then
   sudo umount "$BTRFS_MOUNTPOINT"
 fi