xDS Server automated testing

[youngnick]

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.

[danehans]

xref: https://github.com/miekg/xds as xds client prior art.

[danehans]

@youngnick pinging you on this since RC2 is 16 days away. It would be a big help to the project if you can still take this issue on. If not, can you document how you would resolve this issue, and maybe someone else can take on the issue?