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>
class Banner_Banner_Block_Monblock extends Mage_Core_Block_Template change to
class Banner_Banner_Block_Monblock extends Mage_Adminhtml_Block_Template.