Plugin aware extensions should also be reset at each test setup

Plugin aware extensions should also be reset at each test setup

PluginAwareExtensionManager caches extension according plugin.
At each db plugin test setup, testing plugin and router are newly
created.
However, the extension manager held the extensions for first time
loaded plugin. Because of their cache.

So if a plugin conditional extension exists, it could not be loaded by
testing order.

Fixes bug 1043381.

Change-Id: Ic994aab61c3d67aa9d422594484049f41ff8b321