tests: Fix data race on vptr

Merged Hugo Beauzée-Luyssen requested to merge chouquette/medialibrary:fix_vptr_race into master

When tearing down MediaLibraryWithDiscoverer, we end up stopping the parser from the media library destructor (not the MediaLibraryWithDiscoverer one) which causes the vptr to be updated to the MediaLibrary one. This will cause the regular getParser overload to be used if the discoverer thread asks for a parser when stopping. By explicitly stopping the discoverer from the test class, we ensure the proper vptr gets used until the discoverer thread joins

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