Sometime plugins need to modify tests. Consider this scenario:

Core (psuedocode):

test "a thing" do
assert Hosts page should have one menu entry containing "Hosts"
end

This will pass, as we have just "All Hosts" by default. Now add the Discovery plugin, and this test will fail, as now we have "All Hosts" and "Discovered Hosts"

In general, we need a way for plugins to tell core that certain tests no longer make sense while the plugin is present - we assume the plugin itself will contain tests for the new/modified functionality