[Edit (May 2020)] - This issue has been reportedly addressed in newer releases of NUnit. Please see Nunit.ThrowsAsync. (Ref this answer, thanks @James-Ross)
This is an example from documentation:
var ex = Assert.ThrowsAsync(async () => await MethodThatThrows());
ThrowsAsync
async
await
https://docs.nunit.org/articles/nunit/writing-tests/assertions/classic-assertions/Assert.ThrowsAsync.html