[wasm] Disable threading tests in System.Threading.Tasks.Extensions

[mdh1418]

When disabling tests with async Task, about half the tests would be skipped. Instead of doing so, an assembly level skip will be added. This PR looks to make the System.Threading.Tasks.Extensions test suite pass on WebAssembly.
Tests run: 0, Errors: 0, Failures: 0, Skipped: 0. Time: 0.004518s

Contributes to #38422

[ghost]

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[stephentoub]

I have the same feedback on this that I've had on other such PRs. This is effectively disabling all of the meaningful tests for this library, and doing so in a noisy manner. If the problem is that "async Task" tests aren't supported, we should instead invest in adding support for that, and until it's supported, just disable the whole library's tests.

[steveisok]

@stephentoub We'll disable tests for this whole library and others that fall into the same category.

Separately, it would be helpful to get your perspective on how we might fix async Task for (single threaded) wasm. My assessment has been to change the way the tests are written, which is a bit too costly. However, I am not an expert here and likely may be missing something.

[stephentoub]

Where is the code for the runner?

My assessment has been to change the way the tests are written

Change how? If you're testing asynchronous functionality, what is it you'd do differently?

[steveisok]

The runner is here (@akoeplinger please fill in more detail if I miss anything)

https://github.com/dotnet/xharness/blob/master/src/Microsoft.DotNet.XHarness.TestRunners.Xunit/ThreadlessXunitTestRunner.cs

And following your post is something I've found worked (for some tests that were previously failing)

https://devblogs.microsoft.com/pfxteam/await-synchronizationcontext-and-console-apps/

[stephentoub]

And following your post is something I've found worked

Then why can't that approach be built into the runner?

Fundamentally, that's what needs to be done: rather than blocking waiting for the task, hook up a callback to the task and use whatever mechanisms are available to pump until that callback is invoked.

[stephentoub]

Separately, it would be helpful to get your perspective on how we might fix async Task for (single threaded) wasm

I worked with @akoeplinger on this today. He has a solution. Let's stop putting up PRs that disable every async Task test until that lands :-) Thanks!