Assertion improvements

[sandermvanvliet]

This PR adds the option to assert that messages with a certain pattern have been logged:

InMemorySink.Instance
   .Should()
   .HaveMessage()
   .Containing("some pattern")
   .Appearing().Once();

which will match test some pattern message.

You can now also verify that any message has been logged:

InMemorySink.Instance
   .Should()
   .HaveMessage()
   .Appearing().Times(3);

or that no messages have been logged:

InMemorySink.Instance
   .Should()
   .NotHaveMessage();

or that a specific message has not been logged:

InMemorySink.Instance
   .Should()
   .NotHaveMessage("some message");