Make WaitForCall extension accept matching calls made before waiting

[rickykaare]

These improvements will make the .WaitForCall() extension method more useful, as it will recognize calls already made to the substitute and return immediately if there is a match.

Before this change tests using the WaitForCall would need to start the wait early to ensure that all calls were captured regardless of timing. Like this:

var waitTask = substitute.WaitForCall(x => x.CallToWaitFor());

await sut.ExecuteAsync(canellationToken);

await waitTask;

With the changed behavior, the WaitForCall can be called "inline" with the test like this:

await sut.ExecuteAsync(canellationToken);

await substitute.WaitForCall(x => x.CallToWaitFor());