Support fine grained control over event dispatch during testing

I added this code to my local version of mage-test to allow me to better control event dispatch during the execution of testcases.

It is pretty easy to use, and it uses observername to control which observers will fire during event dispatch. In combination with the disableEvent / renableEvent methods added previously, this allows great control over event dispatch.

```php
<?php
// this will allow the named observer to receive events
// calling this method will put eventdispatch into a 'whitelist' mode which means only observers explicitly
// whitelisted will have events passed to them
Mage::getConfig()->whitelistObserver('mnspromotion_publish_doubleclick_campaign');

// this will explicitly disable the named observer. In blacklist mode, all events not explicitly blacklisted will 
// receive events
Mage::getConfig()->blacklistObserver('catalog_add_topmenu_items');

// does what is says on the tin..
Mage::getConfig()->disableAllEvents()