test: Fix btrfs FS on CS10 image issue

MR https://gitlab.com/redhat/centos-stream/src/kernel/centos-stream-10/-/merge_requests/2009
already added "CONFIG_BTRFS_FS=m" into config

Signed-off-by: Xiaofeng Wang <henrywangxf@me.com>

Author: henrywang

contrib/packaging/configure-rootfs

@@ -26,7 +26,8 @@ else
   fs=
   case "${VARIANT}" in
     composefs*)
-      btrfs=$(grep -qEe '^CONFIG_BTRFS_FS' /usr/lib/modules/${kver}/config && echo btrfs || true)
+      # CONFIG_BTRFS_FS=m has been added into CS10 kernel config
+      btrfs=$(grep -qEe '^CONFIG_BTRFS_FS=y' /usr/lib/modules/${kver}/config && echo btrfs || true)
       fs=${btrfs:-ext4}
       ;;
     *)