libvirt.tests: Add test for commands in virsh itself group

Add test case for command 'cd', 'pwd', 'echo', 'exit' and 'quit' in virsh interactive mode.

# virsh help virsh
 Virsh itself (help keyword 'virsh'):
    cd                             change the current directory
    connect                        (re)connect to hypervisor
    echo                           echo arguments
    exit                           quit this interactive terminal
    help                           print help
    pwd                            print the current directory
    quit                           quit this interactive terminal