tests: sysinfotest: add new test case for S390
    
    Let's add a test case for S390 with CPU frequency information available.
    Test data is sampled from an IBM z13 system running kernel 4.14 on LPAR.
    
    Reviewed-by: Boris Fiuczynski <fiuczy@linux.vnet.ibm.com>
    Signed-off-by: Bjoern Walk <bwalk@linux.vnet.ibm.com>