There issue with block type `banner/Monblock` .it should be in `banner/monblock`.Block class  type should be small letter.

Also Suggest `used Mage_Adminhtml_Block_Template class instead Mage_Core_Block_Template`  because `Mage_Adminhtml_Block_Template is add more functions` which will `suitable with backend`

The contend after  `</test_adminhtml_index_index>` should with in handler

    <reference name="content">
                 <block type="banner/Monblock"  name="afficher_monbloc"
                      template="banner/afficher.phtml"/>
                       <block type="banner/adminhtml_grid" name="banner" />
          </reference>