Add a test to CompatibilityServerTest that tests a server with the SASL DIGEST authentication. This test will use a client that sends all the correct fields, but change a value in the response field slightly.

To know the correct field values that client can use in this test, take a look at the testRfc2831example1 test that has those hard-coded. You can use the same values, only change the response field. Make sure that the server will throw an exception indicating that the proof sent by the client was not valid when the response field differs.

Test has to make sure that the exception thrown contains the "invalid proof" message