ipatests: replace ad hoc backup with FileBackup helper
    
    Test test_smb_mount_and_access_by_different_users was failing with message
    ```
    kdestroy: Permission denied while initializing krb5
    ```
    
    This happened because the previous test
    `test_smb_access_for_ad_user_at_ipa_client` was calling the fixture
    `enable_smb_client_dns_lookup_kdc` which was doing backup of krb5.conf
    in a wrong way:
    - mktemp (to create a temp file)
    - cp /etc/krb5.conf to the temp file
    - ...
    - mv tempfile /etc/krb5.conf
    
    This flow looses the file permissions, because mktemp creates a file
    using the default umask, which results in -rw------- permissions.
    The copy does not modify the permissions, and the mv keeps the
    permissions from the source => /etc/krb5.conf now has -rw-------.
    
    Fixes: https://pagure.io/freeipa/issue/8115
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>