sbus_codegen_tests: free memory context
    
    The memory context was not freed and therefore a destructor that
    closes connection to D-Bus and performs cleanup task was not executed.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2347
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>