TLS13AuthenticationTestCase currently tests the case where TLSv13 is enabled on the both the server side and on the client side and so the HTTPS connection is established using TLSv1.3.

It would be good to add more test cases to this class. Some examples of cases to consider are as follows:

• Server side has TLSv1.2 and TLSv1.3 enabled, client side only has TLSv1.2 enabled. The connection should be established using TLSv1.2.
• Server side has only TLSv1.2 enabled, client side has TLSv1.2 and TLSv1.3 enabled. The connection should be established using TLSv1.2.

Take a look at the SSLContextBuilder configuration in the test. Notice that the setCipherSuiteSelector(CipherSuiteSelector.fromNamesString(CIPHER_SUITE)) needs to be called to explicitly enable TLSv1.3. Without this, only TLSv1.2 is enabled by default.