Add integration test for X509 authentication

Examples from other drivers:

• https://github.com/mongodb/mongo-csharp-driver/blob/master/tests/MongoDB.Driver.Tests/AuthenticationTests.cs#L323
• https://github.com/mongodb/mongo-python-driver/blob/master/test/test_ssl.py#L489-L525

This should be added for 3.12.x and master branches.