Revert "Also disable tests/charset since it uses btrfs."
    Revert "Disable all btrfs tests."
    
    This reverts commit 1cdb67d6f246cb84b52e59fe51e3def177db9b1f
    and commit 8dddac6b1a6afcdb022f3af479bcbc2ce8e238a1.
    
    A patch has been pushed upstream (libguestfs 1.19.49) which
    works around the btrfs bug in the kernel.