tests: fix plugin loading
tests: fix plugin loading

- plugins_loaded is a better hook than muplugins_loaded
- also, the plugin has to be activated by hook, not after the whole
core is loaded

noplanman/podlove-publisher,podlove/podlove-publisher,podlove/podlove-publisher,noplanman/podlove-publisher,katrinleinweber/podlove-publisher,katrinleinweber/podlove-publisher,podlove/podlove-publisher,noplanman/podlove-publisher,noplanman/podlove-publisher,podlove/podlove-publisher,katrinleinweber/podlove-publisher,katrinleinweber/podlove-publisher,podlove/podlove-publisher,podlove/podlove-publisher,noplanman/podlove-publisher,katrinleinweber/podlove-publisher