Write tests for the workspace switcher block

`testGetWorkspaceSwitchLinks()` is supposed to test `WorkspaceManager::getWorkspaceSwitchLinks()`, but we're actually never calling that method on `WorkspaceManager` here. We're only calling `getWorkspaceSwitchLinks()` on the negotiators, but we should test that elsewhere.

So let's change the `foreach` loop and just call `WorkspaceManager::getWorkspaceSwitchLinks()` instead and test the return value of that.