testutils: Explicitly name virTestCompare*() arguments
    
    Currently, some arguments are called strcontent and strsrc, or
    content and src or some other combination. This makes it
    impossible to see at the first glance what argument is supposed
    to represent 'expected' value and which one represents 'actual'
    value. Rename the arguments to make it obvious.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
    Reviewed-by: John Ferlan <jferlan@redhat.com>
    Reviewed-by: Andrea Bolognani <abologna@redhat.com>
    Reviewed-by: Ján Tomko <jtomko@redhat.com>