Add analyzer favorizing Async assertions

[stan-sz]

For a test code similar to

Assert.ThrowsExactly<SomeException>(() => foo.BarAsync().GetAwaiter().GetResult());

add an analyzer rule to favor

await Assert.ThrowsExactlyAsync<SomeException>(() => foo.BarAsync());

also adjusting the test method signature, if needed.

[Evangelink]

I think you can enable CA1849 or [VSTHRD002]https://microsoft.github.io/vs-threading/analyzers/VSTHRD002.html)

[Youssef1313]

CA1849 is unlikely to be triggering here. It only flags call when already in async context. But maybe VSTHRD0002 indeed does. It's unlikely though that it will be able to provide a codefix.