Create alternate GuardClauseAssertion that supports methods marked with `async`.

As discussed in #268, the GuardClauseAssertion class does not wait the tasks to complete, making it unable to validate non fail fast methods.

This is completely understandable, since the fail fast is indeed the best option. Nevertheless, it seems impossible to implement fail fast behavior when dealing with methods marked with the async keyword.

[Kralizek]

@zvirja @aivascu could we reconsider this? I really don’t see why in 2021 async methods can’t be tested properly.

[zvirja]

@ploeh Basically, any async method is not fail-fast. Consider the following example:

public async Task<T> GuardedOpenGenericMethodReturningTaskResult<T>(object arg)
{
    if (arg == null) throw new ArgumentNullException(nameof(arg));

    await Task.Yield();

    return default;
}

This method doesn’t throw if you call it will null. Instead, the failed Task is returned. Therefore, the GuardClauseAssertion it its current form fails and complains about missing guard clause.

In real application almost all the code is async, therefore each invocation is awaited and exception is thrown immediately. So it’s completely OK that “fast-fail” principle is violated in a form that you should await Task to see exception.

Of course, you can re-write code somehow to make it fail immediately (create private function with async keyword and move all the logic there), but there is no practical sense to do it.