Update to swh.core 3.6 and better tests for masking and blocking proxies

Use the new swh.core.config.get_swh_backend_module() instead of the local implementation used in get_storage().

And test masking and blocking proxies can be initialized.

Check these 2 backends can be initialized using swh db commands using bith the backebnd syntax (:) and the full module name (eg. storage.proxies.maslking).

This requires swh.core 3.6.1 which includes a fix for these.