xds: Fix data race

TestEnvoy.Close used e.stream.recvCh == nil to indicate the channel had already
been closed, so that TestEnvoy.Close can be called multiple times. The recvCh
was not protected by a lock, so setting it to nil caused a data race with any
goroutine trying to read from the channel.

Instead set the stream to nil. The stream is guarded by a lock, so it does not race.

This change allows us to test the agent/xds package using -race.

.circleci/config.yml

@@ -237,7 +237,7 @@ jobs:
           command: |
             mkdir -p $TEST_RESULTS_DIR /tmp/jsonfile
             pkgs="$(go list ./... | \
-              grep -E -v '^github.com/hashicorp/consul/agent(/consul|/local|/xds|/routine-leak-checker)?$' | \
+              grep -E -v '^github.com/hashicorp/consul/agent(/consul|/local|/routine-leak-checker)?$' | \
               grep -E -v '^github.com/hashicorp/consul/command/')"
             gotestsum \
               --jsonfile /tmp/jsonfile/go-test-race.log \

agent/xds/testing.go

@@ -189,7 +189,7 @@ func (e *TestEnvoy) Close() error {
 	// unblock the recv chan to simulate recv error when client disconnects
 	if e.stream != nil && e.stream.recvCh != nil {
 		close(e.stream.recvCh)
-		e.stream.recvCh = nil
+		e.stream = nil
 	}
 	if e.cancel != nil {
 		e.cancel()