[#1251] Fixes ResourceProxy tests and add some new ones
[#1251] Fix TestPlugins.test_plugin_loading_order

We used to check if the plugins are loaded in the correct order by removing the
last loaded plugins from the array. That used to be 2, but as we created new
plugins, this isn't true anymore.

To make the test more stable, I've changed it to not depend on the number of
plugins loaded before the ones we're interested in testing.