test_ca: add empty index.txt.attr file
    
    Although is does not harm because 'openssl ca' creates the
    index.tx.tattr file with a suitable content automatically this patch
    adds the file to the test_CA directory to silence a message like:
    
    Can't open ./index.txt.attr for reading, No such file or directory
    139867607979840:error:02001002:system library:fopen:No such file or
    directory:crypto/bio/bss_file.c:74:fopen('./index.txt.attr','r')
    139867607979840:error:2006D080:BIO routines:BIO_new_file:no such
    file:crypto/bio/bss_file.c:81:
    
    which is show by recent versions of OpenSSL.
    
    Related to https://pagure.io/SSSD/sssd/issue/3436
    
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>