Calling BlockDisplayVariant::setConfiguration() won't change results returned from plugin collections

BlockDisplayVariant uses plugin collections for the list of blocks and selection conditions. It implements a special getConfiguration() to mix the plugin collections' configuration into the main plugin configuration. However, it doesn't do the reverse on setConfiguration(), which means if you change a block configuration (for example) in setConfiguration() it won't actually change the list of blocks returned!

This is basically about making setConfiguration() to the same as we're already doing for getConfiguration().

This fixes tests on the page_manager wizard issue: #2550879: Use CTools Wizard API to add/edit Pages (and move plugin UI using PluginWizardInterface)