Example tests for Solid Notifications and WebSocketSubscription2021 specifications

[edwardsph]

This is some provisional work on testing these notifications related specs. Of note:

• All tests are disabled by default for all test subjects by skipping tests with the @notifications tag.
• The tests in protocol don't specify a subscription type as they are meant to test the protocol, not a particular subscription type. Since the discovery mechanism is not yet specified subscription-endpoint.feature uses an alternative implementation and defaults to WebSocketSubscription2021. It also contains code which should allow it to pick the first available subscription type after notification channel discovery but this is untested as there are no implementations available.
• It is not clear to me what https://solid.github.io/notifications/protocol#server-subscription-access-controls is supposed to test so, for now, this test contains an example of getting a notification from a container. Although it doesn't specify a subscriptionType the actual code is using a WebSocket.
• There needs to be a lot more testing on notification events but we are waiting for requirements to be defined.
• This is not built to test the solid-0.1 protocol since that is insecure and will not be retained in the Solid specification.

Comments on this would be very welcome.

[sonarqubecloud]

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[edwardsph]

See solid/notifications#141 re implementations

[csarven]

I suggest closing this WebSocketSubscription2021 or at least updating significantly towards WebSocketSubscription2023.

Also update to work towards https://solid.github.io/notifications/protocol , which has a few things towards 0.3 that may be better off to focus on than the released 0.2 ( https://solidproject.org/TR/2022/notifications-protocol-20221231 ). But generally okay to approach 0.2 - just refer to specific URIs as usual.