xds_test: Wait for server to start serving in RBAC test

[arjan-bal]

Fixes: #8286

This PR makes the test wait for the xds server to enter SERVING mode, otherwise it closes the net.Conn without returning any error.

grpc-go/xds/internal/server/listener_wrapper.go

Lines 287 to 297 in 763d093

	if l.mode == connectivity.ServingModeNotServing {
	// Close connections as soon as we accept them when we are in
	// "not-serving" mode. Since we accept a net.Listener from the user
	// in Serve(), we cannot close the listener when we move to
	// "not-serving". Closing the connection immediately upon accepting
	// is one of the other ways to implement the "not-serving" mode as
	// outlined in gRFC A36.
	l.mu.Unlock()
	conn.Close()
	continue
	}

Analysis: #8286 (comment)

Testing

Verified the test passes 10^5 runs on forge.

RELEASE NOTES: N/A

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.31%. Comparing base (515f377) to head (7c642b1).
Report is 10 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8287      +/-   ##
==========================================
+ Coverage   82.14%   82.31%   +0.17%     
==========================================
  Files         417      419       +2     
  Lines       41344    41904     +560     
==========================================
+ Hits        33961    34494     +533     
- Misses       5957     5960       +3     
- Partials     1426     1450      +24     

see 50 files with indirect coverage changes

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[easwars]

I don't know how many tests that use an xDS-enabled gRPC server have this boilerplate that creates a listener, a stub server, a gRPC server, and then starts serving. The stubserver.StartTestService is very convenient for tests that don't need an xDS-enabled gRPC server. Maybe we can have a helper for the tests that need an xDS-enabled gRPC server, similar to stubserver.StartTestService? (and maybe we can get some TVC to work on that?).

[arjan-bal]

setupGRPCServer in this package is a helper that creates an xDS enabled gRPC server. I changed the test to to not use it because I wanted to set a server option for setting the serving mode callback. The helper was already setting this server option. I made the setupGRPCServer accept a variadic list of server options, similar to StartTestService. Callers can override the default options set by setupGRPCServer as setupGRPCServer appends the passed options to the end of the list used to create the new server.