Address ConnectionListener.AcceptAsync nullability in tests

[CarnaViire]

ConnectionListener.AcceptAsync() was allowed to return null in #41304.

There's test code here that implements this API:

runtime/src/libraries/Common/tests/System/Net/VirtualNetwork/VirtualNetworkConnectionListenerFactory.cs

Line 56 in 8b5467e

public override async ValueTask<Connection> AcceptAsync(IConnectionProperties options = null, CancellationToken cancellationToken = default)

This code is used in Connection tests, possibly elsewhere.

I assume this didn't break because it's just a nullability change, but we should fix it regardless. And we should fix the behavior of the test code to return null when disposed and validate this.

[ghost]

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