[wasm] Skip System.Net.WebClient test suite

[MaximLipnin]

Part of #38422.

Actually System.Net.WebClient test suite crashes with the error described in #38807. But since WebClient class is not recommended for new development, I just skipped the respective tests on WASM.
Besides that, I updated System.Net.WebClient with throwing PNSE on wasm.

[ghost]

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[akoeplinger]

Looks good but I think we should also add PNSE for System.Net.WebClient and point people to HttpClient.

[stephentoub]

Looks good but I think we should also add PNSE for System.Net.WebClient and point people to HttpClient.

The async APIs on WebClient should still work, right? It's just the sync ones that won't?

(I don't have a strong opinion here; if we want to PNSE the whole assembly because it's legacy and not recommended for new use, ok.)

[akoeplinger]

@stephentoub yeah that would probably work but I just checked and WebClient never worked in the mono/mono-based WASM so I think we should be fine with marking it PNSE here.

@MaximLipnin could you please add that? With this message: System.Net.WebClient is not supported on this platform. Use System.Net.Http.HttpClient instead.

[stephentoub]

yeah that would probably work but I just checked and WebClient never worked in the mono/mono-based WASM so I think we should be fine with marking it PNSE here.

Ok