xDS Server automated testing

Description:

#176 introduces an xdstest command, which instantiates an xDS server to perform manual testing against. This issue covers work to turn this into an automated test that can be run by CI or locally.

The largest part of this work is that it will require an xDS client test library that supports:

• connecting to an xDS server
• accepting updates
• Comparing the updates to an expected outcome

Because this process is asynchronous and requires multiple goroutines working together to work, there's a bit of design work to ensure that the testing framework isn't its own source of bugs.