test: change channel operations to avoid potential goroutine leaks

In these unit tests, goroutines may leak if certain branches are chosen. This commit edits channel operations and buffer sizes, so no matter what branch is chosen, the test will end correctly. This commit doesn't change the semantics of unit tests.

server/etcdserver/server_test.go

@@ -213,7 +213,7 @@ func TestApplyRepeat(t *testing.T) {
 	// wait for conf change message
 	act, err := n.Wait(1)
 	// wait for stop message (async to avoid deadlock)
-	stopc := make(chan error)
+	stopc := make(chan error, 1)
 	go func() {
 		_, werr := n.Wait(1)
 		stopc <- werr